EXTREME Overclocking - Cooler Master Real Power 550W Power Supply Review - Page: 2 - Tweaking PC Hardware To The Max
According to that, +12V is split into 3 rails, with the 3rd one only supplying 6A
He'd have to put each card on the 1st and 2nd +12V rail. And that would be pushing the boundaries at peak load....
I find it quite annoying how the ATX specification tells manufacturers to use multiple +12V rails. It means you have to specifically balance load between them if you're approaching the limits of its output.
I'm planning on getting a single +12V rail PSU for when I upgrade to 4870's.