Performance wise, Windows XP 64-BIT Edition would be the better choice. But on the usability front, you're better of staying with 32-BIT XP. The 64-BIT version is not very well supported, and you will have a hard time trying to find games and applications that will actually take benefit of the platform. Not to mention that you would lose all of your multimedia codecs on 64-BIT
Mega's correct. In all likelyhood, 64-bit support on the Windows front won't really take off until Vista debuts, and then it would more than likely be aimed at Vista.