Frankly I would wait on it. Just install your 32bit XP and wait for Vista.
The problem is that all your drivers for XP64 need to be 64bit drivers. Meaning EVERYTHING that is a driver, not just external hardware. In other words, you need 64bit drivers for printer, scanner, camera, and software programs that use drivers such as CD burning apps, and even Antivirus.
I think currently only McAfee and Avast have 64bit AV programs on the market.
You can't use CD emulators and so on. A whole mess of things. So you would have to take inventory of every piece of hardware and software you plan to use, and if it has a driver, that driver needs to be 64bit. And if that software has a 64bit version, you need that too.
The second thing is, for regular software that doesn't require drivers, pretty much ALL software minus some titles, is still 32bit and will be until XP64 and Vista64 become more popular. And you will gain NO benefit running regular 32 bit software under XP64.
The only place where XP64 pushes ahead is when everything you run is 64bit. Your computer, XP64, and a 64bit application. Otherwise, XP32 may even run faster.
As for why it's cheaper, I can't say. Maybe the price is low so they can let people expirement before Vista comes out. Maybe they think it won't be sold much after Vista so they are getting rid of it. I don't know.
My advice would be to load XP32 as your primary working OS. And if you like, and want to expirement, load XP64 on a second partition so you can see if your software and hardware will work on it.
There is no main difference between XP32 and XP64 except for the existense of Internet Explorer 64bit version.
Hope that helps.