Sorry for my late answer.
I uninstalled the Nvidia driver and installed it again. But now 3D acceleration doesn't work at all.
So I will make a fresh install of Fedora (because I have no idea what is wrong). And than I will come back to this thread again ...
Licaon, thanks for this link. I have artifacts like in the Counter Strike screenshot and also like in the Guild Wars screenshot (video ram and gpu artifacts). So after following the hints, I have no gpu artifacts at the moment in Windows (but the video ram artifacts are still there).