100% do NOT go with Vista 64-bit. It has huge driver issues. You will regret it.
Except that I have NO driver issues, and I don't regret it at all; in fact, I'm much happier than I was with XP.
I can't tell if totalwar23's post was in jest or not, but it illustrates exactly what I'm talking about. Who cares if you get 650 frames per second or 800 -- if you're averaging 100 or more, any difference is completely meaningless. Older games are going to run just fine on newer hardware in Vista 64 even if they don't run as fast as they do in XP, and newer games will be slightly slower, equal, or slightly faster in Vista. I've seen benchmarks to this effect.
I don't think DX10 is a good reason to switch to Vista, at least not until games start looking better or running faster in DX10, which they haven't yet.
Last I checked, 8800 GT cards were in stock at Amazon.com at the MSRP of $249. I haven't heard anything about a 65 nm 8800 GTS, have you got a link that talks about that? Would it be based on the G92 chip? The reason the 8800 GT outperforms the earlier 8800 GTS in most cases is because of all the internal tweaks in G92. If a G92-based 8800 GTS does exist, you'd get the best of both worlds.
Actually fazz, "try before you buy" is probably the best way to go. Don't listen to me or anyone telling you how much Vista sucks, see if you like it. If it doesn't work for you, drop it like a hot potato.