Well two 5850s are killer and would be much faster than a single 6950 or 6970. And remember that while the flash could work, it's generally risky as the cards maybe be "binned" as having worse components.
But I mean you probably can't sell a 5850 for even $150 since it's used. Let's say $125... So you're spending around $125 more for the 6950.
As someone with two 5850s I don't see why not just CF them and enjoy some excellent performance and upgrade later when it'll be a big jump up like to the 7xxx series next year.
The only reason I'd take the 6950 now is if it's the 2gb model and I was going to do eyefinity, but even then you really need two 6950s to have enough GPU power.