I'd like to add that the main reason I'm confused on which to install is that Windows XP has always been more stable, but might be outdated compared to Vista. Also, I've always read that 32-bit has issues with recognizing 4 gigs of memory, while 64-bit has compatibility issues.
XP 32 bit can recognize ~3.25G of the RAM. So some of it will be wasted.
Vista 32 bit is pretty much garbage.
Vista 64 bit is now a pretty stable system that can utilize all of your RAM.
I use XP. I've never felt the benefits from vista were worth switching. I would say it's your preference b/t the two. There are some structural changes between the two platforms that can take some getting used to.
You could go with what's cheap now, then look into windows 7, which is coming out soon.