I just got my 6800GS yesterday and when i installed it i was able to play oblivion just fine, the grass didn't look all that impressive but it was ok. Then i installed Half Life 2 and Counterstrike Source and i downloaded the newest drivers for my card from the PNY site. I installed the driver and when i went to play CS, some of the models didn't show up some times. Example: When i would spawn for the round, some of the bots showed up correctly and some of them, only their weapons showed up, or sometimes only the bot showed up with no gun. So i went and turned down all of my settings and tried it again, no difference. I went to the nVidia site and downloaded the newest drivers from nVidia and installed them, no difference. I'm feeling kinda stuck at the moment, and i don't really know what to do with it.
It might be worth a try to go back to an older driver. I just got the eVGA 6800 GS AGP 2 weeks ago, and according to forums, and reviews + my own experience....the 81.98 drivers don't even work. I don't know if this is a 6800GS quirk, or eVGA problem.