750 watts is plenty for that setup... in fact if ure feeling daring... you should be able to squeeze 2 670's (no overclocking!
) into that box with a 750w PSU.
think of PCIx bandwidth lanes as a large freeway. PCIx 2.0 is a 16 lane freeway - speedlimit is 65. PCIx 3.0 is a 32 lane freeway - speedlimit is STILL 65 =). After all electrons in a circuit cannot travel faster then the speed of light.
So would you go any FASTER on the 3.0 freeway? no not really... BUT you CAN handle more traffic. Traffic that todays cards and games just aren't capable of generating.