It looks like this test shows 2 x GTX 580s in SLI at full stress run at 719w. While it also depends on what else you have in your system, I think you should be fine with the 950w.
A quality 550W power supply will handle any single GPU. A quality 750W will handle any double GPU solution. (yes, overclocked on both CPU and GPU and with case fans and other things that normally come with a fully functioning computer).
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