what you have its ok all you really need its a better graphics card and a processor (quad core) and stay away from the last ati cards they are too laud sounds like a blender, the new geforce 580 (450$) or 550 (180$) should be good enough for skyrim, but if you want your system to last at least 5 years more go with a good processor maybe a i5 or i7 and probably you'll need another motherboard which is going to bring the price a little higher cause you need to change ram too.

hope it helps