Here is Guru3D's power supply recommendation:
GeForce GTX 970 or 980 - On your average system the card requires you to have a 500 Watt power supply unit.
GeForce GTX 970 or 980 in 2-way SLI - On your average system the cards require you to have an 800 Watt power supply unit as minimum.
With AfterBurner we applied:
Temp Target 80 Degrees C
GPU clock +185 MHz
Power limiter 110%
Mem clock +500 MHz
Volatge + 87Mv
FAN RPM 70% (remains silent)