The problem you are most likely having is that many games don't fully support SLI and a lot of the time you are still just gaming on a single GPU while the second one is just sitting there sucking up the power. Many tech websites recommend to just run a single powerful GPU as opposed to 2+. I used to run SLI 770s and like you didn't see much, if any difference while gaming. I sold my 770s and bought a 980ti, which is about the same in capability as my 770s were, but with the full time power and not the "if all situations are correct" power. I would go with a GTX 1060 6gb or GTX 1070 if I was you.