In the order you asked:
I owned two 780's a couple years ago (first just one, then a second for SLI), and I was fairly exasperated with how quickly they became obsolete from a high-performance perspective. For me, the 3GB VRAM was absolutely crippling as soon as a game started to exceed that. If you don't care about textures, then you can probably get away with the 780. I have a Strix 1070 now, though, and I'm very happy with it. You have a 4K TV, if you have the itch to upgrade, you may as well get a new GPU and get the most out of your setup. Your i5 is perfectly sufficient for powering a 1070, even a 1080, even SLI of both.