I have a 2000+ and a GeForceTI4200 playing in 1600*1200 with many things on, but i never get more than 30 fps (even in 1280 or 1024).
I guess that's an Windows XP effect as some friend having lower configuration run at 35-40fps under Win98.
But one thing is still strange for me, because depending on the GP4 installation (i reinstalled it dozens of times), speed feeling is different.
I can remind i played with a low sense of speed and then reinstalled it and get a sudden impressive feeling of speed, i was even feeling cars were runing too faster !
But i never found what makes such difference, I can only suspect something with my sound card drivers (SBAudigy). I know this point as already been discuted but this has nothing to do with material sound acceleration as disabling it never got me any more fps.
So maybe reinstalling your game, you will have a different sense of speed...