I'd say going for the upgrade version would be the cheapest. As far as your Windows XP Professional goes, an in-place upgrade should be easy, if not always the best way. However, just remember that with an upgrade disc, a system crash (such as a hard disk failure) will mean that you cannot easily install Vista on a clean system (i.e., on a hard disk with no pre-existing OS). There are workarounds available, but you'd have to install Vista twice Look around for the workaround if you're truly interested.

Cheers,
cgkanchi