Why buy vista when they will release new version soon? 'nuff said.
I don't like all the sneaky back door things that microsoft brought in with Vista, all the stuff to do with file management which means that they can make it you can't access your own files (happened to my friend, he's a professional photographer, but it wouldn't let him access any of his 40,000 photos). I'd say that most of what people think is good about vista (ie graphics) is more gimmicky, while stuff like stability on anything but new machines leaves a lot to be desired (and I don't even like the new look of vista, but that's just personal preference). I'm voting xp with this one.
High end rig for gaming go Vista for DX10 and general better looks ect.
Mid low end machine for web browsing ect go XP no need to take the performance hit of running Vista unless you think Looks > All
Or wait for the new Windows released late this year early next.