At least I wouldn't install xp 64bit. It feels just like a quick fix from Microsoft before they can release Vista. 32bit systems work very well already and the 64bit windows won't bring much if any more speed and not all software is ready for that. It's just the larger memory amount which it can handle.
Btw, isn't the athlon64 X2 processor a 64bit processor? Buy one of those and a good mobo and you'll be fine with both 32 and 64 bit operating systems.
Btw, I don't even know which processes work at 64 and 32 bits in my mac right now but I don't really care. The only thing which matters is that it just works.
