The Seasonic X-Series 660W Model X-660 (SS-660KM Active PFC F3), with its +12 Volt continuous current rating of 54 Amps and four 6+2 pin PCI Express Supplementary Power Connectors, is enough to power a system using two Radeon HD 6950 graphics cards in 2-way CrossFireX mode.
You'll also be happy with its full modular design.
It should be able to handle any overclocking you may wish to attempt. That power supply is over-engineered so it should be able to deliver over 750 Watts on its DC Outputs.
I read reviews on power consumption of video cards. The good reviews do a decent amount of testing and typically include full system power draw at load and idle. The online calculators are crap, they really don't give an accurate number. http://www.anandtech.com/show/4061/amds-radeon-hd-6970-...