The AMD 64 Bit Processors support both 32-Bit code (x86) and 64-bit code
(AMD64 aka x86-64) at full speed. The _current_ version of XP 64 bit
only support Intel Itanium Processors (IA64). Microsoft does have a
beta release of XP for AMD64 in beta. Expected release date is almost a
year away, though. It sounds like they are timing it with the release
of SP2.
Athlon 64 is still a very fast processor on current 32 bit code and will
run the 32 bit version of XP just fine. There is very little software
available for 64 bit yet anyway. NVIDIA did just release their AMD64
bit drivers for the beta, however.