To overclock the Powercolor HD6950 PCS+ Vortex II we used Sapphire's excellent TriXX software tool.
This is easily one of the best software overclocking tools for AMD users.
The card didn't prove to overclock that well, although it can vary from sample to sample. We only got another 30mhz from the core, indicating a 3.5% overclock increase. The GDDR5 memory maxed out at 1451mhz, a 11.6% clock increase. Increasing the cards voltage didn't actually aid overclocking.
The manual overclock helped gain another 2 fps over the ‘out of the box' settings, which is a decent achievement considering the results now basically match the reference clocked HD6970.