7900 GS: NVidia's new midrange
I believe that this article should help you. The 7900 Gt will be a stronger performer than the GS. The Gt has more pixel and Vertex shaders. As far as the companies you have chosen, both are good companies that make good products. As far as your power question, I don't really know. As you can see from my post count, I am not all that experienced. Anyway, you now have my two cents. Good luck!