Depends on the game and how it uses textures, etc.
Just look up benchmarks of fairly common games. Call of Juarez, Bioshock, World in Conflict, etc. etc.
Some games, the 320 totally tanks, but it tanks so bad Nvidia thinks its driver problems with the card (I'm referring I think to maybe Firingsquad benchmarks for Call of Juarez I think).
Consider this -- Crysis demo is supposed to hit the 25th of this month. Wait for the inevitable flood of benchmark results from it, since it's supposed to be the most advanced game in the known universe at the time, and then guesstimate from there!