After days of searching this and other forums, testing and swearing, I think I have a usable GP4 on a Nvidia GeForce4 MX420 card!
Home XP operating system. P4 3.05 MHZ. 512mb ram.
9.6 patch.
Manual in game Graphics setup:
Texture Detail- Low 16bit
Environment Maps-Off
Bump Mapping-Off
Shadows-off
(the above cause CTD if changed)
Smooth set to 6.
Everything else is maxed.
Get 56 Frames.
Have run 6 champ races with no CTD.
Kinda miss the higher textures especially with rain effects but still good visuals.