My 640MB 8800 GTS runs games at 1920x1200 well up to 4xAA and 16xAF depending on the game, I play BF2142, ut2004, Prey, FEAR, Doom3. I get typical fram rates of around 60fps, though BF2142 can slow down to about 45fps if there is alot of action. Additionally for comparison, my dad says that Prey and other games work well with his 256MB 7900GS at 1920x1200 and high settings though I am not sure if he is using any AA or AF. Also, I use XP so expect vista and anything using dx10 to be slower.
I think the 320MB 8800GTS would be good enough for most games today, though I'm not sure how it will hold up with max settings on upcoming games such as UT3 or Crysis. You might have to turn down the settings a bit, I'm not sure if it would be worth it to spend an extra couple of hundred of dollars just to max out the settings.
Hope this helps