first i wanna say "hello" to all members of the techpowerup forum! :)
2 x 6970 = 2 x 250 W = 500 W
500 W + 150 W (cpu, even with OC) + 50 W (board + peripherals) = 700 W
---> your 750W psu shouldn`t be a problem, if it`s not a cheap low-budget-model with a weak 12v-lane.