That PSU is actually overkill for just 2 7950's. The 7950 draws 150w at full load, and spikes never go above 180. with a 62a 12v rail, you can run 3 7950's without even going over the 75% load max efficiency window for PSU's.
just a question.You can run ur system smoothly even with a 600w power supply you bought 750w for a future crossfire?
The motherboard you have is not meant for Crossfiring. Get either the Asrock Extreme 4 or the Gigabyte z77-Ud3h.
That case is very nice. My cousin has that. Its very spacious and runs very quietly.