I've installed a lot of high quality texture packs and the "nice" reflection mod, and still average around 55-60 FPS in standard populated areas of track. In solo areas it has been known to fly up in the region of 85-100
It does drop to about 25 if I'm at the back of a full grid. And if there are a lot of people causing a lot of smoke it can drop even lower.
AMD Sempron 64 2800+ (ASUS KVM-MX mobo)
1Gb RAM
GeForce FX5500