With the R9 295x2 pulling 500w through its 2x8 pin connectors, I cant see any reason why the Fury x2 would need more than that...The 295x2 use's two fully featured AND overclocked 290x GPU's on it...
Reports suggest the Fury X will use 250-275 watts of power and run at 50°c under load, with the 2x8 pin connectors there to allow for huge overclocking headroom.