well, 7 is newer and just as good as xp,even better. so, pretty simple answer. but you'll want to make sure all your hardware and peripherals have drivers for win 7.
Personally, if you're going to be doing gaming on it, go with XP. Some games, in my experience, have not worked with Win7 and I'm pretty pissed I blew all that money on Ultimate with little support.
You could also do a dual boot if you wanted to, just one idea.
I run a dual boot set up for studio and internet stuff.
I recently installed W7 64 bit Ultimate on the internet install and I love it. Dare I say it, WTH, it's better than XP.
I only play one game and it works fine and even though I can't comment on it's gaming abilities i'm seriously thinking about installing it on my studio install and running my audio software via W7 too.
Personally, if you're going to be doing gaming on it, go with XP. Some games, in my experience, have not worked with Win7 and I'm pretty pissed I blew all that money on Ultimate with little support.
Pretty much. 64-bit can handle larger amounts of RAM better which allows for better performance when running multiple programs or memory intensive apps.