I wanted to do the same, but i believe there are several applications that wont work on XP64, plus you will need to get drivers for all your hardware....the normal ones dont work. If im wrong, please tell me because im dying to upgrade. I hear its quite a bit faster.
64bit allows you to address more RAM. At present there aren't huge performance increases, also it depends what software you use (render stuff, encode video and music, play games, run word processor software, etc.)
Nope. I think Microsoft only became serious with 64-bit Windows when they released Vista (think Microsoft released 64-bit XP to stop AMD from nagging them, when AMD released their first 64-bit CPU).
For Vista 64-bit you'll get drivers in the future, but XP 64-bit, I'm afraid not.