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 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(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.
Here are the official power requirements for the brand new ATI Radeon HD 5850 and HD 5870 video cards.
ATI Radeon™ HD5850 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended
600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode
ATI Radeon™ HD5870 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended
600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in • dual mode
The power supply recommendations are for an entire pc system.
Corsair, PC Power & Cooling, and Seasonic are some of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Lately we've been seeing a few other brands offering some high quality units.
The OCZ ModXStream 700 watt power supply is not exactly a high quality gaming psu. It was designed for use by typical consumers and normal temperatures. For ordinary use the unit is acceptable.