I oc'd My i5 750 to 4.0GHz and there was no difference (~10 points - fluctuation) compared with 3.2GHz.
Which tab of GPU-Z? Which Graphics card? etc.
The tab at the bottom left that should have a drop down for both cards. Make sure the bus interface is running at x16 2.0, or at least x8 2.0. Most cases x4 2.0 will kill performance by as much as 40%.