SLI means twin video cards linked with an SLI bridge. Note that most vendor sites will be very conservative on wattage ratings since a quality PSU and a "steenky" PSU could both have the same wattage sticker on them regardless of ehat they actually do.
Pop over here to have the thing "do the math" for you. The GFX card itself draws 94 wattshttp://www.antec.outervision.com/
You don't ned anything high end for your intyended use so I'd pop over to http://www.silentpcreview.com/Recommended_PSUs
Skip the Editor's Choice list and pick something off the "Recommended List". I'd suggest a Corsair VX series.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003