Here is Guru3Ds power supply recommendation:
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.
If you are going to overclock GPU or processor, then we do recommend you purchase something with some more stamina. http://www.guru3d.com/articles_pages/asus_geforce_gtx_660_directcu_ii_top_review,7.html