First of all, thank you for your replays.
After playing with nvidia settings, and using need for speed profile, fps raised up to 50-60 when I'm alone on the track. The problem is when there are more cars (AI): Starting grid 20 fps and then 30-40.
What about the best settings for the 6600gt card(AA,AF,...)?
When playing the gpu temperature is about 85ºC and cpu 70ºC.
I think I'll have to get a new cpu (athlon XP 3200).