Never buy a MS OS until the 3rd SP.
Or better yet, just buy a Mac...
Win7 IS Vista SP3, period. It's trimmed down a little and runs a little better. I like it better than I liked Vista (not saying much). Runs applications good.
But if you like mucking around in the system as I do, XP is still the way to go, if you do Windows at all, that is.
I'll end up using Win7 eventually, of course...