Microsoft is a bit behind in its 64-bit support!
But having said that, the Athlon 64 runs 32-bit stuff well, so you'll be fine with the 32-bit XP.
I also have an Athlon 64 and it dual-boots into either the 32-bit version of Windoze XP or the 64-bit Ubuntu Linux.
It is a bit odd, having a 64-bit CPU and not being able to make the most of it from within Windows...
- Ali