$80 for a hd6950 thats a very nice deal considering that a new or refurbished hd6870 costs close to $110 or $120, with hd6950 costing quite a bit more. There have been rumors that it does not work on all cards, and of course if the card has a dual BIOS it is a safer. If your card is one of the earlier 2GB hd6950's there is a good chance that it will work. However there is always a risk that you completely brick the card. So IF you decide to flash it make sure to make a backup of the original BIOS which in some, not all emergencies can help you recover your card if something goes wrong.
I nor Tomshardware take NO responsibility if you damage your card, so PROCEED AT YOUR OWN RISK! And obviously this will void your warranty, if the card still has one.
An alternative to flashing the BIOS if you dont want to take that risk is to simply overclock the card, but that obviously does not unlock the shaders.