I recently bought my GTX 660 TI, with it Skyrim runs perfect in 1920x1080. Every setting is set to max, AA 8x, AF 16X, Ambient Occlusion on and set to Quality.
That Card might be out of your budget, but the GTX 660 without the "TI" is much cheaper while not being that much slower, and the prices are dropping constantly.
Whatever Card you buy, make sure it has at least 2 GB of Vram, i beat this limit very closely, thats why i bought an Card with 3 GB of Vram.
Even with only the first Highres DLC installed the Vram consumption is over 1 GB, add some mods.....