You would notice no difference unless you are running a network of 40+ computers. The 64-bit version of Windows XP is lacking a lot of the features found in the 32-bit version. Among the missing features are: DVD playback, the ability to record to a CD, the Kodak imaging accessory, Windows Media Player, NetMeeting, IEEE 1394 audio, and fax capabilities. The 64-bit version of Windows XP is also missing almost all protocols except for TCP/IP, System Restore, the Windows Messenger Service, Remote Assistance, the File and Settings Transfer Wizard, and too many other features to list.
Personally I think it is a total waste of time for home users but each to their own.