(1) Use XP 64 if you like, but know that it's LESS supported than Vista 64. Not more. And whether we like it or not, Vista *is* the OS that Microsoft is going forward with. So this situation will only stagnate for 64 bit XP, not improve.
(2) "Quad Core" processor has little effect one way or the other because your APPLICATIONS still have to be written to use multiple cores. Just because you're using an OS that's aware of 4 cores - (XP 32/64 and greater) - doesn't magically change this. The game that only knows how to run on 1 core will still *only* run on one core. Since very few consumer applications are multi-core "aware", the advantage is that different apps can be run on different cores. So your game can have a processing core all to itself, while other tasks run elsewhere.
Therefore (3) What exactly do you need this to do? WHY do you need a 64 bit OS?? Are you looking to Game, want 4GB+ of RAM, and therefore want a 64 bits??? If so, I'd recommend you do some research on how well games run on XP64 before spending your cash.
Oh yeah.... (4) Why are you asking this in the Vista forum?