have you got at least directX 8.1b? and are you trying to play the games with 32bit colour depth? if so step back down to 16bit and see if it helps. run dxdiag to see if it comes up with a problem and make sure your detonator drivers are the signed versions if you are running xp.
and make sure theres nothing much running in the background, also try giving your gfx card more space move any pci cards further away