I also have to agree that 670's in sli is your best bet. As was mentioned before, only by heavily overclocking 680's in sli will you get a better deal.
Same could be said for 7970's in CF. If you opted to overclock them (keep in mind they're likely louder, hotter, and more power hungry than 2 680's) you would actually get even better performance than 2 680's in sli, for a little more money than 2 670's.