I dunno if you can upgrade your windows xp straight to windows 64bit but it should since all the previous windows did have an option to upgrade, if not just partition an hd or get another one, n' install that sucker in multi boot. I use grub for that since I got fedora core 3 going. Pwnage. I'mma wait on FC4 after they get maybe another couple of upgrades onnit.. or when someone burns me it since I have limited bandwith... so whichever happens first. :P
Now, have you already bought the x64 pro edition of windows xp? If not, I suggest holding back for now. Only Far Cry has a 64bit edition of the game, very few actual windows 64bit programs... so all you'll really benefit from is your windows os will run much faster is about it.. maybe just a couple seconds off the load times, I dunno.. But yeah, if you wait, the price will drop onnit, so that's extra cash in the pocket, or you could save up for that new Windows Longhorn due out if not delayed at the end of next year.
But yeah, ultimately, it's up to you man. You'd be able to show off how uber- fast your os is, and your far cry in respect to gaming (Did I mention I love to game? :P).
DOH! can't believe I forgot to mention this, but if ya want dude, there's free versions of some of the Linux os's that are 64bit. But you do have an ati.. and ati's support for Linux is sadly, quite low compared to nvidia. Don't expect to play anything on Linux without some painful framerate loss.. for now.