You can also get an HD 6850 and it should perform better than the 6790. It will have plenty of graphics punch for Skyrim at the resolution your monitor is. These GPU's are pretty cheap these days. You can even play Battlefield 3 on high settings with this card.
it's $135 after rebate, I would suggest you save up a bit more, this GPU is worth the price. Your 500W PSU would handle this GPU fine.http://www.newegg.com/Product/Product.aspx?Item=N82E16814150573