when it comes to 6950s it is not the brand that counts as much as the design. when buying
a 6950 for mining you want a reference card. that is a card that AMD designed and manufacturers
copied pretty much exactly. it is those cards which can have the extra shaders unlocked and thus
improve performance. they also overclock well and you can control the voltage. plus they have
a dual bios in case you screw up the card you can revert it back.

hope that helps.