The common rule of thumb is a high class 500 to 550 watt power supply with enough current (amps) on the 12 volt rail(s) can simply power a system with any single video card completed. A high quality 700 to 750 watt power supply with enough current (amps) on the 12 volt rail(s) can power a system with two video cards in use in dual mode. There are a few omissions like the new ATI Radeon HD 5XXX series cards which employ less power due to their energy competence.