XP should work just fine on an AMD64 processor, as it runs 32-bit code natively (ie without an emulator ala Itanium).
The only thing a 64-bit OS or software will do is allow you to take advantage of the 64-bit extensions in the architecture. It won't necessarily give you more speed, just more address space.
<font color=red> If you design software that is fool-proof, only a fool will want to use it. </font color=red>
Windows XP could not wok with 64bits
but it could work with 32bit mode because Athlon64 could support 32bit mode
if you want to use 64bit mode you can choose 64bit Linux