Depends on the game. Roughly speaking around 30 or more is desirable for me. Sometimes a game can run at low fps and feel very smooth. Just got hold of Crysis Warhead today and running all bar one setting at 'enthusiast' dx9.c, max res 1680x1050, 8xAA and the fps was from as low as around 10 - 40+. However, if felt and seemed like it never really dropped below 20 - 25. Nevertheless I found that turning off AA completely made little if any negative impact to the visuals and the fps shot up, which wasn't surprising.