Lets say that i get some good ol' ddr2-800 G.Skill CAS 5 RAM.
Can i underclock it to 533mhz (on a core 2 duo), and lower the CAS to 2?
Wouldn't that offer a performance increase as long as I don't overclock?
533mhz vs. 800mhz on a stock setup is an equal contest right?

no, no motherboard will let you to lower the timings beyond 3 on the market that I know of (at least no amd boards do, as I have the top end crosshair and it doesn't let you), but I think that you probably won't be able to run the memory beyoned 900mhz anyway, so run it at 1:1 anyway, and overclock the cpu a bit to makeup for the difference (or lower the multiplier and bump the fsb a bit, as far as I know your warranty is valid as long as the cpu has not gone over the stock clock speed, so you can raise the fsb without problems with a lower multpilier).  And no, 800mhz beats 533 at stock, but if you can get the fsb to 400 with 1:1, that will outperform the same 800mhz with a different divider, but you'll have to lower the multiplier to something like 5 to reattain your warranty

First, i doubt you'll be able to get it down to 2.  You could probably hit 3, maybe 2.5, but I doubt 2.  Sorry, I'm to lazy to do the math right now as to which is faster.  (533@CL3, or 800@CL5)  This also assumes a 1:1 memory ratio, you can change the ratio to allow the 800 ram to run at its default settings.  Whether or not this helps depends on whether the task is memory bandwith starved or not.

 
"[O]n a core 2 duo" is irrelevant, you want to be looking at the motherboard, or more specifically, its chipset.  To answer your question about "533 vs 800", the answer is simply no.
 
When you start up your computer activate the BIOS by hitting F1, F2, F11 the delete key, or whatever the screen says to activate the BIOS.  If you're not familair with it, it'll look really blocky with everything.
 
For my own motherboard I had to go into Chipset something to muck with the memory.  For myself I could set the speed of my memory to 400, 533, or 667 speed, and 3, 4, or 5 for CAS latency.  I'm assuming that CAS 3 is the lowest supported speed for DDR2 RAM, but I could be mistaken.
 
Past tests that I've read about say that for every full stepping of RAM speed (400 -> 533 -> 667 ->800) with a corresponding increase of CAS latency (3 -> 4 -> 5-> 6) shows an improvement of approximately 4% overall.
 
Underclocking, unless for cooling purposes, will be almost pointless.  My opinion anyway.
 
Disclaimer: I know almost nothing about this kind of stuff.  Someone more knowledgeable then me should post and either back-up or refute the above.  For the original poster's safety.
 
Edit: I type too slow, follow the advice above.

Try to find me a motherboard that will let you lower your timings below 3 and then we'll discuss on how to make it possible.  For one thing, so far only ddr ram is capable of running at those timings, and only with tons of voltage and lower frequencies, I haven't ever seen any ddr memory get above 500mhz with cas2 with any settings before, usually they are all at cas2.5 and still suck up something like 2.7v just to remain stable

If I were you, I would download SiSoftware Sandra and do some tests. Under benchmarks, choose the one that says memory bandwidth.   Try different memory speeds and timing ratios until you get the most out of it.  Always follow up sandra with orthos or something similar to test for stability and/or vice versa. Try to find the sweet spot where you get the most stability at optimal memory speed or should I say memory peformance.
 
Cappster

The gist of your question seems to be "can I buy 800cas5 and clock to 533 to lower the cas?" In that case, yes, likely you can.

Yeah, basically that's it.
I'm sorry if I missed it, but IF i did buy 800mhz ddr2, at CAS5, would it be more beneficial to underclock it to 533mhz, which still saturates the bus, and lower the CAS to 3 or w/e, so it is faster in that respect.  Or, would it be better to keep it at 800mhz CAS5?
 
Sorry if i missed the answer to this question in the replies:::

DDR2 don't usually down low to 2cas and lowering from 800 to 533 will slow it down regardless of the tighter timing. Try adding more voltage and tighten the timings at 800 or increase frequency at the same timing.

I see, thanks for the help in my theoretical question.

From what little I know, 800 Mhz CL5 always beats 533 CL3 (unless the timings get silly) To get more general-purpose speed, use the highest CPU multiplier and 1:1 ratio. Then set the timings a low as possible.

From what little I know, 800 Mhz CL5 always beats 533 CL3 (unless the timings get silly) To get more general-purpose speed, use the highest CPU multiplier and 1:1 ratio. Then set the timings a low as possible.

enewmen is right.  See this article:
http://www.madshrimps.be/?action=g [...] rticID=472

Depends on what type of applications you will be running, for synthetic benchmarking and latency related applications, I think the cl3 would perform better

Yeah, but with gaming, I doubt It'll be better, so thanks guys.

Depends on what type of applications you will be running, for synthetic benchmarking and latency related applications, I think the cl3 would perform better
 
You're right. That is why I said "general-purpose speed".  

for gaming, as you said, higher memory clocks will help, but unless you have an 8800series gpu, your gpu will be a bottleneck anyway

yeah, thought so.

What gpu do you have anyway, because unless if you have something extrmely powerful as I said, then save the risk of damaging your ram and don't overclock it that high

Me?
This is all theoretical, WHEN i build my compy I'll probably have a 8800GTS 320 meg or a 8600GT/U.
In my sh1tty Pentium 3 i have an fx5500, which i added for a whopping 30 bucks so I could play quake3.