Well, we've already had this discussion a hundred times in the OS forum, but you're new so I'll just tell you.
There are no advantages in XP64. It was a little experiment by Microsoft to see if they could make a 64 bit OS.
It was never adopted by mainstream home or office users. Its difficult to find software and impossible to find drivers for and even if you get a 64-bit game running, you'll be very disappointed in performance. At this point there arent any gains.
The 32-programs run just fine because all the x86 class 64 bit processors still run 32 and 16 bit instructions. As far as I know XP64 supported most of them. The only challenge was finding drivers, and not too many people were making 64 bit programs at the time.
Vista is what you want. That OS has been designed from the ground up to support 64 CPU's. Lots of folks say it will stink. You can check out the OS section to see all the arguments, some of those folks actually know what they are talking about and get into a little depth during the conversation.