Ive just updated to a x64 XP, and I havent noticed much in the way of incomptability with games (though Ive not had time to extensively check it), and the 64-bit Crysis is definately much better in performance, everything runs on maxed out specs easily enough.
In my experience the extra RAM is definately worth it, but having used computers with similar specs to mine with the same quantity, but faster speed of RAM, I can say that, for gaming at least, you don't need to spend extra on hugely expensive RAM, as long as you have enough of it!