Would I be better off selling my ALMOST new 7850 and putting the money towards something better such as a GTX680?
Would my rig be able to use a 680 at it's full capacity?
You can always use a GPU/CPU at its full capacity regardless of what its mated with. Servers didn't suddenly get slower because they only have a built in 8MB video card.
If you want pure performance and 3D goodness , go with the 670, 680, or 7970