EVGA 01G-P3-1441-KR GeForce GT 440 1024MB (Fermi) DUAL DVI PCI Express 2.0 x16 Video Card $79.99http://www.newegg.com/Product/Product.aspx?Item=N82E16814130612
This card has a minimum psu requirement of 300w so it will run on your system. It will also let you play Skyrim with a fps of about 35 to 40 fps depending on the settings and resolution you choose. If you can get 30fps on a game then it is playable without problems. Since Skyrim scales very well for lower end systems and taxes the cpu and gpu equally to obtain a bit more performance then you could overclock both the cpu and gpu slightly to achieve just a bit more fps. Evga has a nice software called Evga Precision that makes overclocking the gpu very easy. I am not saying you have to overclock to the extreme here but just a bit will make some difference if you need it.