Anyway, if you already have a 6970, crossfire with another and get a slightly fast 7970, or a computer that scores damn close to it. $150~ for another 6970 is better then spending $360~ for a 7970, IMO, and wait for the next gen of cards to come out. And you got to wonder why in reviews of the 7970 they didnt pit it against a 6970 crossfire set up.
I would have picked up a 2nd 6950 but my PSU wouldn't cut it. I'd have to upgrade the PSU and then the cost would be a wash with a 7970.
Also, once overclocked to 1.05ghz+, the 7970 performs very close to 2 x HD6970 ~ HD6990s. But I can see how in some games that are GPU constrained, the dual 6970s would beat it.
I agree with you that bang-for-the-buck, 2x unlocked HD6950s for $300 are a great deal if you can find them used for $150 a piece. Generally speaking on paper two mid-range cards tend to provide similar performance for less or better performance for the same price as a top-tier card (GTX460 OC SLI vs. GTX480). For example 2x HD7870s for
$500 are faster than a $500 GTX680. I think a lot of people would take 20-30% less performance to not deal with dual-GPUs for gaming though.
But ya, if you can't notice micro-stutter, two slower GPUs can be an alternative to a fast single-GPU card. Even when next generation of cards are released, the same argument could be made for 2x HD7950s/660Ti/670 cards against say a single GTX780/8970. I am sure 2 current generation mid-upper cards will beat a next generation single-GPU flagship as well. HD7950 is already dipping to $310, which means it's not out of the realm of possibility to think that 2 of those will cost less by the time a single HD8970 launches.