The above answers are all great but even a couple more things to consider:
1) You said you'll be streaming games too- this makes more work for your CPU. So if you're going from just playing normally on a 960 to streaming and playing presumably a higher frame rate on the 1080, that's going to be a significant jump in CPU power needed. I don't know a ton about streaming but I'd guess for best performance you'd need a fast quad-core like, say an i5-6700 (others can correct me on this, but CPU load in a "powerful card + streaming" setup is definitely something to consider)
EDIT: You added you have an i7-4790K... CPU power shouldn't be much of an issue. Could be if you go over 100FPS while streaming in some games maybe?
2) spdragoo is correct in the general outline of cards with resolution/framerates but that's generally for well-optimized FPS games. Other situations can arise that throw that progression off, for example I wasn't happy with my RX480 (similar to 1060) performance in Mass Effect Andromeda at 1080p/60Hz (had to use Medium settings and still getting some frame drops), and wanted a 1070 but those prices were badly inflated at the time so I went with a 1080. While this is somewhat overkill at 1080p/60Hz (ultra+ settings), it isn't leaving as much GPU power on the table as I thought it would... usually sitting at high-80s utilization and jumping fairly frequently into the low-90s range. (this case is rarer obviously, as I'm going for max eye-candy in campaign-mode games, and ME:A is known as a pretty badly optimized game, but things aren't always totally cut and dry)