I agree that planning an SLI or crossfire as an upgrade option most often is a bad idea. By that point you might as well go with the current gen best single GPU solution.
I got my 970's, both of them, for just over 300 bucks from a friend who decided he wanted to go the 980 route at the time, so I jumped on that (it's been a while now, wow how the time flies), as at the time it replaced the 770 I had been running with. Mine are also the 4 GB OC versions. A single 1070 right now that could replace that would be about 400 on the low end and the performance bump would not be noticeable at all at this point. The aforementioned friend runs a rig now with a 1080 ti and mine can still hang in frame rate on lots of the games we play together, and even the ones he far outpaces me I am still getting 80+ frames. But again in bang for the buck I just can't justify upgrading or building a new rig to get from 80 fps to maybe 90 or 100, or even 110, when the difference is all but unnoticeable, and that is on my most demanding games. On my favorite stuff, basic first person shooters, I get well into the 100's fps. I have priced it out and to build a rig that will really replace mine at this point in time I would be well in excess of 2k, closer to 2500, at the minimum.
So I will hold out for now. Maybe in a couple of years or with the next step change in GPU and processor architecture I can justify it. Or just when I get the itch to build a new system I will go at it again.
