I know Morbid has already posted the settings he has and how he can run GP4 on his system, but I have a very different system to his and so I thought I would show the performance I get.
SYSTEM:
2ghz Pentium 4 Processor
256mb DDR Ram
Geforce 4 mx420
GRAPHICS CARD SETTINGS:
Anisotropic Filtering - 8 x
Anti-Aliasing - 2 x
GRAND PRIX 4 GRAPHICS SETTINGS:
Reflections - off (look crap on and save fps anyway)
Textures - Medium
Pit crew - Medium
Standard view - All objects
Mirrors - No trackside objects
Environment mapping - Off
Bump mapping - Off
Advanced lighting - Off
(any others I've missed at medium)
GRAND PRIX 4 GRAPHICS SETTINGS (ADDITIONS):
Heat haze - On
Video Screens - Off
Arcade and TV overlays - On
Mirrors - On
(tell me if I've missed any out again)
With all these settings, on 800x600 resolution I get 24fps, which I've grown to love, and after time is pretty fast anyway. On the next resolution, 1024x768, I get 22fps. The great thing is, with any number of cars on screen the fps stays the same - no slowdown whatsoever.