According to SmartComputing Magazine the 64-bit version, like duder wrote, is going to be run by more powerful processors like the Itanium and AMD Hammer, AFAIK. However, unless you are into heavy video producing, data crunching/compiling, or something similar, you don't need to worry about an upgrade. The reason the common user doesn't WANT 64-bit XP is because the processor (Itanium, Hammer, etc.) has to emulate 32-bit applications. Of course, this is all based on common users, but if you wanted to try it out, feel free. I actually wouldn't mind giving it a try just to see what the hell the difference is between 32 & 64-bit because honestly I have no damn idea. Hahaha.
Take care man.