GeForce GTX 660 - On your average system the card requires you to have a 450~500 Watt power supply unit.
GeForce GTX 660 SLI - On your average system the cards require you to have a 700 Watt power supply unit as minimum.
Still the answer depends on what model ..... (note: never what brand) .... we are talking about. Generally, manufacturers make PSU's to fit various market niche's ..... some are excellent, some are good and others simply are not. Even within Model series you can have duds. The Corsair HX series for example garner 10.0 ratings from jonnyguru **almost** across the board. The HX1000 and HX1050 however score significantly below their HX series brethren.
1st tier - Antec SG, CP / Corsair HX, AX / XFX Black / Seasonic X* (10.0 jonnyguru ratings most of the time)
2md Tier - Antec HCG, HCP, Cprsair TX, XFX Black (9.5 jonnyguru ratings most of the time)
* That's a sentimental nod to the Seasonic .... scores of late, like the Antec HCG , HCP have slipped to 9.5.