If you have no need for 4GB of memory, and have no 64-bit applications, I would skip x64 for the time being. Even with Vista delayed until 2007 it would probably be better to wait. Drivers for most common hardware should be fairly easy to find (or are already included) but apps may have some trouble. Specifically utilities like AV, firewall and the like. Though there are 64-bit versions available.
Bottom line, IMO; If you have XP 32-bit, just use it. If you need to buy an OS, and don't have any unusual hardware and don't mind a lil work to find applications for security and the like, go for x64. Either way, you shouldn't have to do too much extra.
\Dan