WinXP64 lets you use all your RAM; WinXPx86 (32-bit) can only recognize and use about 3.25 GB of memory (all memory - CPU, RAM, and GPU memory). WinXP64 uses windows on windows (WOW) to execute 32-bit programs and puts them in a folder called Programs (x86).
WinXPx86 gets slightly higher scores in the various game benchmarks than WinXPx64 - probably due to the 64-bit's additional WOW process. I triple-boot WinXPx86, WinXPx64, and Win7RC1x64 on one machine (the top one in my sig) because I wanted to see the differences on exactly the same hardware.
WinXPx64 requires 64-bit drivers for whatever you want to add: printers, scanners, etc., and many older ones have no 64-bit drivers.
-- If you want to use hardware that you've had a while, and you want slightly higher gaming benchmarks, and you can live with paying for ~.5 GB of RAM you cannot use, run WinXPx86.
-- If you want to access all the RAM you bought, use WinXPx64. Another note: I've found that Win7x64 can usually use Vistax64 drivers, but none of those work with WinXPx64.