The main advantage of x64 at the moment is the integer register size of 8 bytes rather than 4. This makes certain mathematical programs faster - useful if you're doing project Euler >.>
Check this
But yeah, 64-bit programs tend to have a bigger memory footprint (pointers are 8 bytes not 4 bytes now amongst other things) so my RAM usage is way up on xp64 compared to xp32.
Also, can't use old DOS / 16-bit installer stuff on 64-bit. Things like C&C and RA1 won't install (although RA1 will run with some patching and stuff).