Well to answer that question, it entirely depends on if your card supports the game. Secondly, think of it like this, if the game is graphically intense, then you know yourself it's not going to max it out, probably on low/medium settings. If you're lucky, some at high or very high.
I have the 9500GT (512MB) and I get around 20-25 FPS when playing Far Cry 2 (Only on DX9, If it's DX10 it lags really bad). Crysis gets around 15-25 FPS if I've set everything on High Settings. If I put the Shader Quality and Particles effect to medium settings, it gives me a performance boost (30 FPS and around 25 FPS when in fights). GTA4, well I dunno, haven't tested it, but from what I seen, it doesn't seem to good to handle IV.
The ATi card seems equivalent to the nVidia card, but like Steam said, go for ATi, cheap.