Right now there aren't many advantages but the 64 bit capability is there, and you're not paying extra for it so why not have it.
An athlon XP 2800+ costs $105 and the A64 2800+ is $124. I guess the improved architecture is worth the $19 so you basically get the 64 bit instructions for free.
Right now there's a 64 bit windows and a only a few 64 bit applications and games, but I think that now that the hardware is available widely the software will follow.
You can get 64 bit linux also.
Nothing is as easy as it looks