Are you running with AA ?
Is your CPU running at stock speed or have you OC'd ?
Don't try overclocking your video card yet. Your lust for max FPS might get the best of you and if you get too aggressive you could create problems for yourself.
You would see better performance gains by running a mild CPU OC.
What MB are you using ?
War had a good point. Check your CPU load when playing one of those two games. See if there are any background applications that are taxing your CPU.