Looks good! I'm sure that PSU is sufficient for that system but the issue is, Seasonic power supplies don't have the notoriety and quality (I think) that a good Coolmaster or Corsair PSU does. Here: http://www.newegg.com/Product/Product.aspx?Item=N82E16817171052&Tpk=coolermaster%20gx%20650
is the PSU I use and I absolutly love it (don't worry about the negative reviews it's great). Plus, then you have the option to add another card down the road.
Imagination may be more important than knowledge but when recommending power supplies at least SOME knowledge is pretty damn important. Coolermaster has a reputation all right .....for the absolute crap they sell as power supplies. Seasonic has a repuation as well.....for making the best power supplies on the market including many of the high end Corsair models.
OP stick with the Seasonic. And a system with a single HD 7850 only needs a recommended 450w unit so the 520 is more than enough for your build.