So how many FPS would I drop?
It really depends on the game but I believe it will be a slight bottleneck. It takes something like the GTX 670 to slightly bottleneck the FX-8120. So my advice, overclock just a bit to get out of the hole you've dug yourself in to before you get caught under piles of jaffa cakes.
(GTX 670 and HD 7970 are around each other in performance).
So just like Tom burnquest has stated, not too much, in fact I'd say little.