So i'm building a gaming pc. I'm not sure if i should go with a 790i board with DDR3 memory or a 780i board or below with DDR2 memory. I'm still new to building a gaming pc and i'm not sure if DDR3 or DDR2 setups are better for gaming as far as performance and fps. I mainly play World of Warcraft and COD4. Would much appreciate someone more knowledgable with memory to help me out.

I say go for a 780i board with DDR2 because the difference between DDR2 and DDR3 isn't noticeable and DDR2 is cheaper too
Also remember DDR2 vs DDR3 doesn't make a noticeable difference in gaming,the main components which make a difference are GPU and CPU so be sure to get a good GPU and CPU

Are you planning on using SLI for your new build? If not I wouldn't recommend an nVidia chipset on your mother board. They have been known to cause problems with overclocking. If you don't plan on using SLI a good Intel chipset x38/48 would be a better choice for a more reliable board.
 
The DDR2/3 question was answered above and I agree. No real benefit right now with DDR3. Just a waste of money.

And because its a waste of money, DDR3 is (currently) worse for gaming.  What does 4GBs of DDR3 cost?  The cheapest 2x2GB sticks of DDR3 that newegg has are these.  
 
http://www.newegg.com/Product/Prod [...] 6820220305
 
Patriot 2x2GB DDR3-1333.  $154 counting shipping and rebates.
 
Here is the cheapest DDR2-800MHz ram that newegg has in stock.
 
http://www.newegg.com/Product/Prod [...] 6820609218
 
Super talent 2x2GB DDR2-800.  $60 counting shipping and rebate.
 
This is a $94 difference.  That dollar amount is nearly enough to bump you from the 4850 to the 4870.  That amount can bump you up a CPU, or get a faster/larger harddrive.  That amount is more then half the amount needed for a Raptor 150GB.  
 
I'm not saying DDR3 is bad, its just to expensive for what you get.  Seeing as it doesn't really pay off to get ram thats faster then your FSB, I'm not sure I see the point of it.
 

Definitely agreed^^

First I would not even bother with an AMD chipset for an Intel CPU, PERIOD.
Next comes my Bang For Buck analogy, DDR3 just don't give you bang for buck. It's marginal improvement just isn't worth the premium price. DDR2 is in historic terms cheap, buy lots of it. Right now the best bang for buck chipset is the P35.  With to me the DFI BloodIron motherboard and Intel Q6600 for a quad setup, with a good 8800GT video card.
When Intels Nehalem, with it's on die memory controller comes out I will have to re-evaluate this.

Go with DDR2 at this time ... you can't find much difference between DDR2 and DDR4 performance compared to the price.

lol dont listen the the guy that said to go with an  8800 gt. lol those things are so old and outdated go with a 4850  $199 < best bang for buck. and if you want to really shine get a 4870 $299.00 and crossfire them
 
and to your question. get ddr2 800 and oc them period

 
Huh?  I didn't realize AMD made chipsets for Intel CPUs.  He was looking at getting an Nvidia chipset, I think you meant that.
 

 
This next bit is all over the place.  The P35 is a great motherboard, and does provide a very nice band for your buck.  But the Q6600 does not.  The Q6600 is $200, while other faster dual core CPUs are less.  The 6850 for example is 600MHz faster, and is $15 less.  If you don't mind overclocking, the 7200 is a very popular choice.  It starts higher then the Q6600, and is a mere $130.  Yes you are giving up two CPU cores, but that doesn't matter much for gaming.
 
The other bit, as already mentioned, is the 8800GT.  If you are going with an P35 CF board, you are better off with a 3870.  Should perform very similar to the 8800GT.  I personally would get the 8800GTS 512MB, as that seems to be the lowest card you should get for a new rig.  You should be able to find one for $150, a great price for a great card.  If you can, the 4850 is a better deal, but not everyone wants to spend so much for a card.
 

 
DDR4?  Uhhhh, yeah.
 

 
I would not overclock the ram.  Set the ratio to 1:1, and put the FSB at 400MHz.  This runs the ram at DDR2-800, which is not overclocked.  Adjust the multiplier as needed to get the system stable.