You have a TDP of ~140W for each card and a minimum system requirement of 450W for one card. So for an SLI, I'd recommend a good quality 750W because you want to leave some power in the tank for the rest of your components and the overclocking capacity. In terms of PSUs, I'd go for the following:
Seasonic X750 750W 80+ Gold Certified Full Modular Power Supply/Seasonic S12G-750 750W 80+ Gold Certified Wired Power Supply
XFX 750W XXX Edition 80+ Bronze Semi-Modular Power Supply
EVGA Supernova 750w B2 BRONZE SEMI MODULAR 80+ PC Power Supply/EVGA SuperNova P2 750W Platinum Fully Modular Power Supply Unit