the X86 one is the 32 bit version. The X64 one is the 64 bit version. Your processor doesn't matter here.
32 bit apps dun run better in 64bit CAUSE THEY ARE 32BIT!!!
Your processor DOES matter. You cant run a x64 xp in pentium 4 for example. C2D and Q are IA64 architecture, which is x86, plus a semi x64 architecture. It has x64 addressing, but with x86 instructions. Look it up on google, but its quite confusing, really, even after reading it a couple of times.
@go4saket
Yes, your proccy will support both versions.
Okay... apparently there are x64 versions of P4, didnt know that. I stand corrected, though. You cant run a x64 OS on a proccy that doesnt support x64.
I definitely recommend Vista X64 even if u have 2 GB ram cause I have used vista x64 with my C2d E6600 and 2 gb kingston ram and found it far ahead in terms of performance than vista x86(32 bit).
Almost all games will run on vista x64 and some will perform better than the 32 bit counterpart.