Something could be wrong with your system, I can run all my games fine including CoD4 on my 8800GT, and by fine I mean 50+ fps nearly all the time. (Usually a solid 60 on multiplay, I have had the dips into the 40's on Pipeline and Downfall though, only maps that cause a bit of slowdown)Personally I dont think the GT is powerful enough to run games @1920x1200 with all the eye candy on. COD4 for example runs not as smooth as I would like. I have recently just upgraded my monitor coming from a 1280x1024 res and the GT was fine at that res. 1920x1200 seems to be pusing it to the limits I think, perhaps its because it only has 512mb of vram?
Either that or the difference between my res and yours is bigger than it seems, I use 1920x1080 so can't see there being a big difference between 1080 and 1200, could be wrong?
Vista 64 and latest WHQL drivers.