Yes, your PSU should handle that GPU fine.
If you want Nvidia, I would go for this. It has 2GB of VRAM for games like BF3, Skyrim that use a lot of VRAM and it's faster than the 6790 and supports PhysX that AMD cards do not. This particular card will run cooler over reference cards meaning that you can OC it further than reference because of less heat output. It's about $172 (Original price about $182) after rebate and has free 2 day shipping, pretty good deal IMO.http://www.newegg.com/Product/Product.aspx?Item=N82E16814162085