If you'll only run valve games(which I don't believe) get a 7900GS. I run them maxed out at 1680x1050.
For more $60 you'll get a much better card like the x1950xt which allows you to run tougher titles with AA and AF. But if you have $260 to spend and you like Rainbow six vegas, GRAW or Oblivion, get the GTS.
There are two or three games where the 640Mb version smashes the 320 at high resolutions.Nobody knows if the upcoming games will need more than 320Mb to store textures, and because of that I'm waiting to make my move.
I own a 8800GTS 640mb and I run it on a Gateway 22" widescreen LCD at 1680x1050 native. Some games wont even support this resolution, just to let you know. The last guy is right to say that if all you want to run is Valve, just go with the 7900. I have this card and I cant play many games with everything MAXXED out, but then again, I dont have a QX6800 either, so I might be bottlenecked there. But with a 5200x2 and 2GB ram, I can run almost anything with AAx8 and AFx4 at 1680x1050.
I'll tell you this: if you can actually tell the difference in graphics between those settings(x8, x4) and max (x16, x8 ) WHILE you are playing a game, you have way too much time on your hands or you are just bs'ing the crap out of everybody, because I have 20/10 vision on a 22" screen and I cant tell a difference.
Well, other than my FPS goes down 50% between the two settings, I shouldnt lie...