WARNING:  Overclocking may void the original manufacturer's warranty and can cause damage to your card if you are not careful.  I am not responsible for any actions you take and this guide is just that, a how to.
 
First I would like to thank Evilonigiri for helping me out with this guide and for teaching me the basics of overclocking my video card (though I've done a lot more since we talked).
 
So for starters, this guide is going to be about overclocking Video cards using Rivatuner and ATI Tools.  They are both great tools for overclocking and will attempt to show you how to use them on a basic level.
 
This guide is primarily about driver-level overclocking.  BIOS Overclocking is, in general, more dangerous and more noticeable by a manufacturer.  It will also not go into Hard mods like resistor pots and the like to increase vGPU and vMEM.
 
So first thing is first.  What tools do I need?  You will need to download Rivatuner and ATi Tools.  The current versions of each are 2.06 (Riva) and 0.27b2 (ATI).  I will attempt to keep these updated with new information.
 
So what are they used for?  Rivatuner is used to adjust your GPU Core and Memory Clock while ATI Tools is used to test and check for stability and temperatures.
 
First thing is first.  Hardware monitoring.  We can't do anything if we don't know our temperatures.  Once Rivatuner is ready to go click on it.  Under target adapter find the arrow.  Click on that and click Hardware Monitoring.
 

 

 
Now once you have it open find the current core temperature and take a note.  After this open up ATI Tools and click Scan For Artifacts;  Run this for about 2 minutes and watch your Core Temperature climb.  After 2 minutes it should be nice and toasty.  Believe it or not this is the point at which your BIOS in the GPU wants to run the card at.  Keep this in your head when overclocking and never exceed it.  Another option here is to monitor the temperatures is to run a full 3D Mark06 or play a game for 15-20 minutes.  Once you leave it find your maximum temperature.
 

 

 
So now we know what temperature we do not want to exceed.  The next question is.. how loud can you stand it?  Open up Rivatuner and click on Customize Driver Settings (If this is not here please refer to the Video Card Tweaker's FAQ).  Now click on System Settings.
 

 
Click on the Fan Tab and on the scroll down box click Direct Fan Control.  Move the slider up to 100% and hit apply.  If this is too loud, continue to back off until it is within a comfortable auditory range.  Once you have found this, click the Save (NOT APPLY ON STARTUP) button under Fan Profile Settings and give it a name.  You can save multiple fan profiles (I have several for different launchers which I will explain later).
 

 
The other option here is to use low level system settings and using automatic fan control.  You can set the minimum and maximum Fan Duty Cycle and target temperatures.  This is another option, however I prefer to just use direct fan control.  PWM and this pisses me off.  I can't stand to hear things ramping up and down but I don't mind a constant buzz.
 
Now reopen the Hardware Monitor and notice how much the temperatures have dropped.  This is another chance to run ATI Tools again and see how warm it gets!
 
Now it's time to go back into System Settings.  This time click on the Overclocking Tab and Enable Driver-Level Overclocking.  Same thing here as far as saving it, by the way.
 
Now it's time to get to Overclocking!  Start moving ONE slider at a time 10-15 MHz at a time and check it for artifacts in ATI Tools.  Don't know what one is?  Here are two examples:
 

 
This is a minor artifact.
 

 
This is a fairly bad artifact, but I've seen worse on my PC.
 
So what is the goal?  First I go for 30 seconds, so I can move up quickly.  If I don't see any artifacts for 30 seconds I will move up another 10-15 MHz.  Once I start seeing artifacts I back down a bit and start waiting for 2 minutes.  Some people consider 1 minute stable enough for gaming, others will say 2 minutes and some will say 5.  Personally I use 2 minutes and then do a full run of 3D Mark06 and visually scan it for problems and monitor temperatures.
 
First you start with the Core Clock.  Once you start to artifact you can try to unlink the clocks (Shader/Core) and see which one is causing the instability (9 times out of 10 it will be the shaders) to increase the core further.  Once the core is good run a full 3D Mark06 as a quick stability test.  Once this is done it's time to get started on the memory.  The same thing applies here but I will say one thing.  It is possible for the system to freeze without showing an artifact in ATI Tool.  This is why I recommend an occasional 3DMark06 test to check for stability (I've crashed it numerous times).
 
Once you find a good overclock with your fan setting already going, it's time to save it and create a launcher.  Click on the launcher tab of Rivatuner and click Add New Item.  Choose regular item.  Type in a name and then click on the check boxes for associated overclocking profile and fan profile.  From the drop down box select the Fan and OC profile you have already saved.  Now click ok.  Now you have a quick launcher.  Whenever you want to apply the overclock just right click on Rivatuner in the tray and hit the launcher.  The other option is once you are done go back into System Settings and hit Save to apply the overclock/fan profile at the launch of Windows.
 

 
You have successfully overclocked your video card!  There is a lot more to do from here but these are the basics.
 
So what is this thing called PCIe Frequency?  This is the bandwidth lane between the PCI Express Slot and the Northbridge of the motherboard.  The current PCIe1.0 Standard is 500 MB/s in each direction.  This gives you a total of 8 GB/s on a 16x PCIe Slot.  Assuming you are using the standard 100 MHz PCIe Frequency increasing this by 15% to 115 MHz can give you 9.2 GB/s bandwidth.  It is recommended not to OC this past 120 MHz as it can cause SATA Corruption.  PCIe2.0 doubles this to 16 GB/s in a 16x lane.  There is not a card that currently maxes out the PCIe1.0 bandwidth, however when there is increasing your PCIe Frequency may let you enjoy a new graphics card without having to fork out for a new motherboard.
 
PCIe Frequency can be changed in the BIOS.  There was a rumor at one time that raising the PCIe Frequency could result in higher overclocks.  In my personal experience I have not found this to be the case, your mileage may vary.. however for myself, raising the PCIe Frequency did not result in any tangible improvement.
 
Currently in nVidias there is no way to change voltage going to the GPU without a hard mod.  A hard mod is soldering a resistor pot between two points on a video card.  I won't go into how to do it but I just want you to be aware that this is out there.  This is also possible on ATI cards.  Please keep in mind that increasing the voltage will increase heat and can cause electromitigation within the GPU.
 
You can also BIOS flash your card.  Again I won't go into how to do it as that is beyond the scope of this guide.  However you need to be aware that it is possible.  BIOS Flashing can make the stock clocks of your Video card higher without the need to use Rivatuner.  I highly suggest using a completely stable overclock when flashing and testing it thoroughly first.
 
Suggestions are welcome and I will work to keep this guide up to date.  It may go through revisions as I edit it, and in general, attempt to make it better.
 
Other Useful Tools:
 
Real Time HDR Demo - This is also useful for Video Card stressing and temperature testing.  Personally I use this and Prime95v255 on a waterloop that includes the CPU and GPU for maximum temperature testing to ensure a safe overclock.
 
Speed Fan - A very useful temperature tool.  If you follow Computronix's post on calibrations for C2D and C2Q it can be extremely effective.  It also has useful monitoring and logging tools that Rivatuner's Hardware monitor does not feature.
 
AGAIN: READ THIS.
 
WARNING:  Overclocking may void the original manufacturer's warranty and can cause damage to your card if you are not careful.  I am not responsible for any actions you take and this guide is just that, a how to.

Well done, very informative  .

---------------
Q6600 (lapped IHS+ HSF) @ 3.55Ghz 1.425v (BIOS) TT-120
Foxconn 8800GTX(648/1560/2000)
http://www.computerbase.de/bild/news/18649/2/ <==EXCITED!
Fan roundup: http://www.xtremesystems.org/forum [...] p?t=170224

Nice Guide, took me like 3 hours to learn the whole thing talking to Evilonigiri..

May i ask how to change the PCIe from 100Mhz to 115Mhz? Id like to give that a try. And Evil im done overclocking to da Fullest! 680Mhz Core, 1395Mhz Shader Core, 525Mhz Memory Bus. If i go anymore i will start getting those yellow things on ATI tools. Thats pretty good for a GeForce 8500GT right?

 
For the 8500 GT it's not needed at all, BUT.. It's in the BIOS of the motherboard (if it's there).

You can have a little yellow artifacts, they will usually never show in games. That is a very good OC, and by increasing the PCIE frequency to 115MHz, I bet you can hit 700MHz.
 
The setting should be in the bios.

This is also pretty good for testing for stability/artifacts together with atitool (But not at the same time obviously):

Real-Time High Dynamic Range Image-Based Lighting

http://www.daionet.gr.jp/~masa/rthdribl/#Download

Also get this Speedfan that when used in together with rivatuner monitor seems like a more complete tool, because it will record minimum, maximum, and average temperatures of the GPU from the moment it was  executed, seems like a pretty good tool to have besides the monitor (Even though I use both).

Speedfan 4.33

http://www.almico.com/sfdownload.php

EDIT: Also remind people NOT to use fraps while artifact scanning or they might see yellow stripes that can be confused with artifacts.

I think ATI tools is more powerful, if you get what I mean.

Definitely, but it isn't bad by any stretch to make sure that the thing is 100% stable with another small test. Also remember that you can increase the stress on the graphics card by upping the Res and changing settings with this app.

 
Agreed.  I actually use that demo with Prime95 for temp testing a water loop.
 
I prefer ATiTool because it shows the artifact very clearly.
 
As far as Speed Fan go I was trying to keep it simple and use as few programs as possible to avoid confusion.  Speed Fan isn't needed, so it wasn't included.
 
Good points all around.  

Nice.

Something ba has happened with the whole deal.I let it find the max core,but aborted after 4 hours,and it then detects crazy stats as default and i get low scores on 3d mark 06.sm2.0 dropped fro 3100 to 900.

I restarted and sm2.0 is up to 2451,but still omething has gone wrong,y all that sudden drop?

What card??

ATI HD3870.
 
I had the omega 7.12 drivers installed,and when i selected set core,the core dropped to 389 no matter what.Now i installed the oficial ati 8.1 catalyst and sm2.0 is back and better than ever.3200+.
 
But what was with that drop?I still got the omega shortcut group,i didnt uninstall them,i just installed the latest ati.
 
Is there any damage to my card?