Vista all the way. XP was fine, but to me, Vista is like a nice face lift to XP, a few added features, like it actually closes a program that is not responding right away. It pops up these little boxes that prevent anything from getting onto your computer without you knowing, which can be a pain sometimes, but it's worth it.
But I have dual core with 2GB of ram on my laptop
Maybe with less, as people have said, it's not worth it for Vista.
Now don't get me wrong, I would do a nice Linux over all of them if it wasn't for the need for practicality.