I give a bit of an overview as to why Intel is a viable choice
HERE. Take a look. I also provide links to CPU information that you can actually trust, and a current price list.
The Athlon XP does not have 64 bit architecture, as far as I know. I have no idea where you got that information. You're probably thinking of the Clawhammer. And the XP has nothing to do with Windows XP. You've gotta love misleading consumer marketing.
The XP stands for Xtended (or Xpanded, I forget) Performance. But you can bet they only picked that acronym so people would associate the new chip with the new version of Windows. Now, they get to ride on the coattails - for free - of one of the biggest computer product launches ever.
But no, it won't run Windows XP any better than another fast chip. The XP designation denotes a desktop Athlon made with the new Palomino core (as opposed to the Thunderbird core), which has been used for mobile Athlons for a while. It's a smaller micron manufacturing process, and thus can be scaled to slightly higher clock speeds (which they are going to hide from the consumer, anyway).
If you're planning to buy a new CPU, do your homework. There are a lot of reasons to pick AMD, and there are a lot of reasons to pick Intel. But asking people which is better, is not likely to get a response you can trust.