Here is a comparison between the 770 SLI and a 970
http://www.anandtech.com/bench/product/1064?vs=1355
For you 670 SLI, you can subtract 12 - 15% from the 770 SLI figures. You will see that the 970 is better in most games. It will get even better with subsequent driver updates, while the 670 is a mature card and will not get significantly better. So, go for the 970.
Also, remember that a single card solution gives you smoother and more consistent performance than a dual card.