Well lets say the GTX 680 almost beats a 6990, which is effectively two 6970's in most games. GTX 680 is almost double the performance, and you could probably overclock past that point as well.
6970 Crossfire is good, but a 680 would probably suit your needs more in that case, although you can also get a 670 and overclock it there as well. A good factory overclocked 670 is = to a stock 680. I would recommend a 670 or a 7970.