For a system using a single Radeon HD 6870 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 30 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
For a system using two Radeon HD 6870 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
The Corsair Builder Series CX600 V2 (CMPSU-600CXV2), with its combined +12 Volt continuous current rating of 40 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 6870. It is also insufficient to power your system configuration with two Radeon HD 6870 in 2-way CrossFireX mode.
The XFX ProSeries 550W Core Edition Full Wired (Bronze) (P1-550S-XXB9), with its combined +12 Volt continuous current rating of 44 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 6870. It is also insufficient to power your system configuration with two Radeon HD 6870 in 2-way CrossFireX mode.
The XFX ProSeries 650W Core Edition Full Wired (Bronze) (P1-650S-NLB9), with its combined +12 Volt continuous current rating of 53 Amps and with two 6-pin and two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 6870. It is also sufficient to power your system configuration with two Radeon HD 6870 in 2-way CrossFireX mode.