You do not need a 1200 watt power suply for an overclocked gaming pc with two ATI Radeon HD 5870 video cards in Crossfire mode. That is total overkill and a complete waste.
Here are the official power requirements for the 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 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four 75 watt, 6-pin, PCI Express® power 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, 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four, 75 watt, 6-pin, PCI Express® connectors for ATI CrossFireX™ technology in dual mode.
The power supply recommendations are for an entire pc system. Manufacturers tend to overestimate poer requirements because they know there are people who will buy low budget, low quality psu's of questionable performance and value.
Here is a link to Seasonic's newest high quality, high performance power supply, the X650 Gold which earned an 80+ Gold certification for energy efficiency:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151088
Here are links to technical reviews:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=169
http://www.silentpcreview.com/Seasonic_X650