The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily power a pc system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the 12 volt rail can power a system with two video cards operating in dual mode.
High quality 750 watt power supplies such as those offered by Corsair, PC Power & Cooling, and Seasonics would be excellent choices. They consistently earn high marks in technical reviews. They are stable, reliable, and come with 5 year warranties.
BTW - The brand new Corsair TX750 has earned an 80+ Silver Certification for energy efficiency.