Actually, (being a smartass

) XP can handle 4gb. However, this is the maximum memory adress space; it includes RAM, VRAM and page file. Say 512mb video memory + 512mb page file used leaves 3gb RAM useable.
64-bit vs. 32-bit doesn't make a difference; the game is 32-bit. To see a difference, the application needs to be 64-bit as well.
As always, graphics hardware is the main issue.