Also whilst the 6950 appears to effectively hash higher than a 7950 (also at greater initial expense), it is for all intensive purposes a dual card board. It has two gpus effectively running in crossfire. You can run more than two 6950's in a rig, whereas you can run 4x 7950s, if that floats your boat...so there is greater potential per rig with 7950s...
Nope, the 6990 is the dual gpu card, 6950 being a single.