Well, the x64 would be better. It allows for more things to be loaded into virtual memory and allows applications to load faster and perform better.
Here is some info: http://www.microsoft.com/windowsxp/6.../overview.mspx
You can find more by poking around their site.
You have to check your drivers for all of your hardware just in case they don't support 64 bit. Also some programs like nortorn I heard don't work.
My friends brother has the 64 bit. But when he plays games like FPS's the controls get stuck sometimes. I have known a perfomance boost (he has a 3800+ and 2 gigs of dual channel, hard to see improvements).
If you have a free version of XP I would give it a go and save some cash. If not then I would go x64.
Wait, don't you have to buy the x64 edition with hardware?