Look at how much you want to spend now, vs how much you want to save for later (full upgrade)
Lots recommend a 1080Ti or just a 1080, but I would look hard at the 1070 as well. For half the price it's not a bad deal, plus the funds to your build account will be replenished a lot quicker.
I would wait and get a top tier video card when you build your next PC. That said, the 770 is only one of the two things holding you back. The other is the location of your game. This game needs to be on an SSD. I would recommend it be played on an M.2 card, but an SSD is the miniminum for performance these days. Spinners may be able to claim transfers at the same speed, but in reality, the read/write for them can't compare to the SSDs. In fact, I would recommend that before you get your new video card, try playing it first with SC on the SSD. I suspect you will be pleasantly surprised. In fact, you may even be able to hold off until the next gen processors and video cards become available/more affordable/more reasonably engineered.
The problem with SSDs is longevity. They have improved over the years, but not enough IMO. I think the next few years will have significantly better longevity (it's already there in the more expensive ones), and this will consign spinners down to archiving and very low priority tasks such as holding your iTunes repository.