If you are willing to spend 100 bucks to go from a gts 320 to a gts 640 just spend another 100 and get a gtx. The performance gained from a gtx is far greater than the performance gained from 320-640.
G9's are coming in november...but seeing as how this is your first build and you just cant wait to assemble it like most first timers go for the GTX it should run things smooth as butter. You should also look into cooling.
Well we're still guessing at the g92's coming in november. Also there is no guarantee that they will be affordable and/or have decent drivers when they first launch. Pesonally i would suggest just getting a gts 320 from evga and waiting, because as of right now, there isnt any game on the market that the 8800 gts cant play at high res.
Vista Ultimate. Why do you need the Ultimate version, and in 64-bit? Home Premium is fine. If you are going 64-bit, make sure that you can find all the drivers beforehand, for the software you'll be installing.