Well ..... the official line from Nvidia (assuming you are using Nvidia) is
For your SLI system, you should have a power supply that can supply 30A from the 12V lines.
NVIDIA has a certification program that qualifies power supplies (SLI-Ready) that ensure 500W total output, 30A on the 12V rails, and 2 PCI Express connectors for your dual graphics cards.
The certified Coolermastern PSU is the 'Real Power 550 SLI' one
Personally I reckon it should be ok ....
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