Recent AMD drivers give a flickering image when using Sapphire Trix or MSI Afterburner (OC software).
So in this point in time, if im right its impossible to Overvolt your GPU, unless you manually edit your Bios to allow higher stock voltage.
So in short, All you can OC is with the Ati Catalyst ControlCenter (Your Driver panel),
It is only possible to adjust the Core speed and Memory speed.
But without the extra voltage, you are very limited in Ocing the hd6950,
Just up your Core/Memory MHz by steps of 5-10MHz, and backdown one step when you crash in Games/Stresstest software.
Without overvolting its allmost impossible to damage your GPU, Even when your PC crashes of to High GPU OC, just reboot system and turn it down one step and test again.
I think you will max out around 850MHz on the Core, and 1325MHz on the memory.
(Very old hd6950 can OC the memory to 1400MHz+, but back in those days they had hd6970 memory chips).
I dont know if you did this allready, but there is a high chance you can flash a Unlocked Bios on your card, giving the Core the shader ammount of the hd6970.
Take a look at this link: http://www.techpowerup.com/articles/overclocking/vidcard/159