well, that's probably part of your answer right there.
Those games are more CPU dependant than anything. Overclocking a video card only helps when you are fillrate limited.
If you want to see if it makes a difference, try running at the highest resolution you can, until it gets choppy. Use these settings, overclock your card, and see if you get an improvement.
You've got to be sure you're maxing out the video card, or video card overclocking doesn't do anything.