Here are the ATI official power requirements for the 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 or greater 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 or greater and four 6-pin connectors for ATI CrossFireX™ technology in dual mode
The power supply recommendations are for an entire pc system. Figure about 40 amps on the 12 volt rail(s) for Crossfire mode.
The Corsair HX650 has a single large 12 volt rail rated at 52 amps. It also comes with the required four, 75 watt, PCI-e, 6 pin, power connections. It can easily handle a system with two ATI Radeon HD 5870 video cards operating in Crossfire mode.
Last January Tom's Hardware published an article about actual power consumption during gaming seessions. There have been similar reviews at other reputable web sites. The results are pretty consistent. Most people think they need more power than is actually required during hardcore gaming sessions.
For example, while playing World of Warcraft an Intel Core i7-920 @ 3.33GHz based system with two 5870's operating in Crossfire mode drew a maximum of 430 watts at the wall outlet (mains). That's for the entire system. That's significantly lower than results for OCCT or Furmark stress testing. There isn't any game that can come close to producing the stress that OCCT and Furmark testing can.
Here are the links to technical reviews of the 5870:
http://www.tomshardware.com/review [...] ,2422.html
http://techreport.com/articles.x/17618
http://www.techpowerup.com/reviews/ATI/Radeon_HD_5870/
http://www.hardwarecanucks.com/for [...] eview.html
http://anandtech.com/video/showdoc.aspx?i=3643
http://www.xbitlabs.com/articles/v [...] d5870.html