In response to your original question, I have achieved over 90 FPS(avg) with my current setup and 1680X1050 and high settings with 8xAA (No VSync). As for minimum FPS, over 30 is playable, 50-60 stops all forms of studdering from occuring. Over 60 FPS and your just wasting energy.
I like to play around high 60's -70's myself. Though on some quick races if you select 15 (max) cars and race at dusk it dips down to the 40's. Racing games benefit from as many fps as you can get , for a better feel. It makes quick steering ,pushing,sliding much more 'realistic' its not a matter of how many fps your eyes can see.
actually it lags in replays when there are too many cars on the screen.also another problem is that when one car bumps into another i observe micro stuttuering like the game freezes for a fraction of a second and again starts.
Lower AA.Set it to 2x.Not more than that.The conditions in which you experience stuttering are normal, nothing to be alarmed of.And you definitely don't need a graphics card upgrade.The 5850 is quite capable at that resolution.