To overclock the XFX HD6950, we used the Sapphire TRIXX overclocking tool.
We use this all the time and it is one of the best pieces of software available for AMD users.
The card proved to overclock well, maxing at 900mhz core and 1558mhz memory (6232mhz effective).
The manual overclock helped push the frame rates to over 51fps, from just under 48fps. A significant increase for such an intensive test.