Why do you recommend 7950's over 7770's? Is it to make more efficient use of the PCIe slots? The 7770 is better at Mh/s / $ and MH/s /W than the 7950 by the numbers.
Oh, and that's one heck of an overclock to get 250 average out of a 7770. Stock is around 200. (at least that is what I get out of mine with poclbm kernel. I got 150 with phatk).
And, depending on what you pay for electricity, you will be lucky to pay for electricity much less get your investment back.