Ooookay, most systems running a 32-bit OS have 3-3.5GB usable.
Because 64-bit OSes eat up more RAM, it's pretty much a wash which gives you more RAM if you have 4GB. You're not gaining anything with a 64-bit OS really.
And 64-bit XP was never something most people should use. It was mostly a testbed for future OSes. If you want to use 64-bit, you want Vista (and if you're buying an OS new, I'd get Vista regardless, though not necessarily the 64-bit version).