The comments about IA-64 and x86-64 were to the thread starter.
As for your question, if you're running the OS in "64-bit" mode, you need 64-bit safe drivers. If you're running it in 32 bit mode, you're probably OK with things as is (although I would then question _WHY_ you're getting an Athlon64, as it's just an AthlonXP like that).
IIRC, there's also a hybrid mode that lets you use 32-bit apps properly, but I BELIEVE you still need 64-bit safe drivers to use it. At least ArsTechnica implies this in their article on x86-64 when they say that you need a 64-bit OS.