I currently run WinXP SP3 on a Athlon XP1700+ with 768 meg RAM and a Radeon 9250 AGP (running at x 4) graphix card.
In game, I run 32 bit sky, high res shadows and textures, no anti aliasing, and 2x AF, with Z buffer depth of 16.
With that I get fps of high 30's when on track myself, but drops to about 8 fps when on start grid with 3 or 4 others, and sub 5 fps when more cars are on grid.
This means I can only realistically race couple of other people before it becomes impracticle as I'm likely to cause a pile up.
Within next week, I expect to get new pc (i7, 6gb RAM, Nvidia GTX480) so fully expect to max everything out and not to have to worry about low fps again!