Expected Mhash/s a bit low?

[RainieDays]

Hi,

I'm new to BC mining and I have a rig with 2 HD 6850s (CrossFireX disabled) running GUIMiner.

Both GPUs are GigaByte brand and have been factory OCed to 820 MHz and memory OCed to 1050 MHz

According to online sources, https://en.bitcoin.it/wiki/Mining_hardware_comparison, I should be getting about 250 Mhash/s from a 6850 but currently I am getting only 150 Mhash/s from each of my GPUs

What could be wrong? 

Thanks

[Xandrah]

Have you installed the GPU SDK?