Well, I'd say download and run driver doctor in safe mode and get rid of any and all old nvidia drivers, then go download the latest from nvidia's website (not microsoft update).
If that doesn't work, there are many other things to try, but my guess is that it isn't your pipelines, as you've 16 of them, and even the 6800vanilla only has 12 and beats a fair amount of games into the ground
Depending on your system specs, you should be pulling down a score of around (maybe a bit under) 5000 in 3dmark05.