There's no issue running in 64bit though is using 32bit processing. As for RAM, do you really need that kind of ram? 4gb ram is really not worth buying and then super rig using i7 or C2D? i7 u need different RAM and different mobo and whether COD will use the new chipset to its fullest is also another question. If you are building a super rig, then make sure u know the main usage of your super rig. You might end up not using the extra ram or speed.

I'm running Ultimate 64 with 8GB DDR2 1333 RAM plus 1kva power, 24 inch LCD, 3TB HDD and OC quad core to 4Ghz with tuniq tower cooler and finally full tower Armor Suite with 4 X 140mm fans, er 4870X2 as well which I think is quite super already.

Make sure u put your money at the right place.

Recommended power supply is 900watts and above true power and certified tri-SLI/Crossfire.
Wondering after getting the super rig, will you get ori games to supplement it?