7850 for sure, make sure its 2GB. Moderately modded I get around 1.4-1.9 GB of vram usage. Also, Skyrim is also mainly CPU, not GPU. Its almost impossible to get 100% GPU usage, as Skyrim also I believe only utilises two cores from the CPU (Thats what happens when you buy a game from Bethesda).
Honestly with my 7850, I'm on like 40 FPS and its pretty heavily modded, and then I look at my GPU usage and see it only on like 60%. Kind of sucks, especially when you can get more frames.