The problem is two-parted. First, I have to admit your card is probably too old, even my PCI-express card (Geforce 6) would require to set the graphics to low. Secondly, your AthlonXP is hardly better then a Pentium 3, if it is any better at all.
By the way, racing games usually take more of your pc's power than shooters like CoD. Because of the high speed, the objects along the track need to be "drawn" comparatively quick. In shooters there is more time available to load the textures.
