You'd want AMD64 Windows right now if...
you need more then 4gigs of RAM.
you want a extra 6-8% boost of performance in certain games.
you work with digital content creation were you applicatiosn that deal with very large memory datasets.. Like if your doing non-linear video editing with high resolution HD formats. Having it 64bits could possibly improve performance and reliability if the applications are specificly designed to be '64bit'.
Other then that not much at all. 98% of the applications you could ever possibly find for Windows XP would be 32bit and are going to be for a while now. Some games, like Ut2004, have native 64bit versions (been able to run those in Linux for a while now), but that's the exception.
So essentially your going to have a 64bit OS that will run 32bit applications vs 32bit OS running 32bit applications.
Personally if Microsoft released a 64bit version I'd get a install disk for it if possible if it cost nothing, and then upgrade the next time the OS begins to crust over and needs to be reinstalled. In another year or 2 your going to see many applications that would be able to run native in 64bit and thus make switching worth my while.
In reality I'd bet that you could setup 2 identical machines, with 32bit on one and 64bit on another and not be able to tell much of a difference at all, except that the 64bit version's drivers suck slightly more for the next few months.