what resolution are you using?
Using 4xs makes a night/day difference in nascar 2000, so maybe you have some problems with your drivers. By the way nascar2000 has its own aniso-fsaa settings so maybe it overrides rivatuner settings.
Try using det 30.30 with rivatuner 11.1, it may solve your problem and also you will be able to tweak aniso performance settings under D3d.
Also check "Force AA in all D3d applications" and disalbe multisample masking.
Sometimes I hate being right everytime.