what resolution are you playing at?
for 1080p, a second 670 will likely get you slightly MORE performance than a 780ti
going higher in resolution, you'll be more and more limited by the 2GB of memory
For the forseeable future I will be sticking with 1080p. One of the criterias for my choices was that I would eventually get more monitors but in all honesty that probably won't happen anytime soon.