How did they convince you to buy those cards? Not trying to be a downer, but they are overpriced for what you get.
For mining? 6990 is one of the best for price to performance. The difficult part is finding one, as it is sold out almost everywhere.
I wouldn't say they are the best "bang for your buck" card. They are 730$ each for 830 MH/s; but you could get 3 5850's for 450 (3*150), they each hash @ 300MH/s, totaling out to 900MH/s....
true, but you can do 2 x 6990 on the same rig, or even 3x. Then the $/Mh becomes much more favorable.