If you want to make sure you have enough to SLi... can always use a calculator.
https://images10.newegg.com/BizIntell/tool/psucalc/index.html
Nice. But, you'll also have to consider the amperage on the 12 volt rail of the PSU. Regardless of how powerful your PSU is, if it doesn't meet the amperage requirements of a graphics card, it may prevent your desktop from powering on. I learned that the hard way last week when testing out an old, SLI/Crossfire ready Ultra 500 W PSU on my newly built desktop. My GTX 650 - which is a fairly low-end graphics card, required 20 A on the 12 V rail. But, because my Ultra PSU was only capable of delivering 16A, I couldn't run it, and was forced to buy an entirely new PSU.
I'm very happy with the circumstance though, because I was able to pick up a much better OCZ ModX Steam Pro 700 W modular PSU. It met the amperage requirement at 25 A on both 12 V rails, and came 80 plus certified (while my old Ultra PSU didn't have any certification).
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