AMD Radeon HD 6950 Lower frame rates

Hello,
I recently Purchased a XFX 6950 1GB and noticed I wasn't getting the same frame rates many bench-markers are getting. They say Dirt 2 should get 70 fps maxed but I only get around 35-40 with 2x anti-aliasing. I have had similar lower than expected fps in other games. Why are the frame rates lower than others are getting? Do I need to upgrade any other components to achieve higher frame rates? I run games at 1080p on my 23 inch monitor.
My Specs:
Windows7 64bit
AMD Athlon II X4 620 OC@2.84GHZ
XFX AMD Radeon HD 6950 1GB
4GB DDR3 RAM
ASUS M4A785T-M/CSM Motherboard
Apevia 500W PSU