64bit, for one it reads all 4gigs of my ram. Second, I got a FPS improvement. Third, everything else seems faster. Fourth, I need to stop talking like this, =].
Currently the best performance is going to intel, and it doesn't look like that will change for a while, so I'm using intel. I have a Core 2 Quad overclocked to 3.005GHz.
Okay well I have an 8800 GT, but I know the 3870 is a bit better than my card. I can tell you with almost 100% certainty that it can max out TF2, CS:S, COD4 and probably more. Its a powerful card =D