Well, you can run the game in 16-bit color instead of 32 in order to slightly improve the FPS. You can also lower the texture quality of everything except for the cars (I run the world/track settings on Medium, Cars on High), turn off the track lighting/shading option (improves performance), turn off the solar effects, and have the drawing distance lowered to help performance (I have it at 43%).
Be sure to check 'number of sounds' down to 8 (or whatever minimum is), and make sure you have no programs running in the background, while your racing. Anti-virus can really impact your fps. Especially Norton. A free ap' called Game Booster(?) can make sure everything turns off, when N'03 is opened.
Running CCleaner, and then Eusing (repeat each until they quit finding stuff) and defragging can also ensure there's no parasitic loss due to malware or fragmented files.
ShadowNight is telling you straight -- more you turn off/down...
...more fps you'll get if you're down on grunt.
Popping in a used $25 graphics card, would likely give you a the type of increase you're looking for.