At this point there really isn't much you can do for an upgrade. Your CPU is still about as good as it gets for gaming. The 6990 is actually faster than the 7970, assuming Crossfire is working properly, so any single GPU would actually be a downgrade unless you are running into huge problems with Crossfire across most of the titles you are playing.
If you want more GPU horsepower, you're either going to have to wait for AMD's next dual GPU card or get 2 7970s if you really want to stick with AMD. On the nvidia side there is the GTX 690, which is two GTX 680s put on a single card, but good luck actually finding one, the only Kepler cards that seem to be consistently available are the GTX 670s, and you'd have to get 2 of them in order to see a performance boost over your 6990.