It's gotta to be your resolution, granted your card is a fantastic model. But still having something running at that kinda res will take the wind out of the best of sails. Also to is your game lagging on online or just singleplayer? Cos your internet connection maybe another factor to consider into the equation. Also too do you happen to put on any Anistropic Filtering or Anti-Aliasing or any other type of rendering? If so try turning it off, also too just make sure your hard drive is defragged and all that kind of basic maintenance stuff.
I know you probably have looked into this stuff, so hopefully this helps you out a bit. If it doesn't then good luck trying to solve the problems, mate.
There are only Two things in life i regret doing... 1. Selling my Soul for Cash.... 2. Buying a Compaq.... I should have kept the Cash, huh.