Eh. Xfire? I'd say that's a no-go. Crossfire still has bugs and issues that have existed several years for all 280X/79xx and below cards. Namely, crossfire with DX9 titles still have microstutter, and crossfire + eyefinity still has microstutter. Ubisoft titles also do not have crossfire profiles with few exceptions. AMD tends to cover AAA titles well, but non AAA titles - a lot of them don't work well with xfire. I would not put much faith in AMD's software team being that this has been a problem with the 280X and below cards for some time. And still not fixed. Years later. Conversely, SLI is very good. Kepler GPUs all have frame pacing working for everything - DX9, surround, etc, and they're good with profiles for all titles. Not just AAA titles. So if you go SLI you'll be good to go.
That said, the 760 isn't a particularly good bargain. I think a 770 would be just fine for 1080p to be honest - i'm not sure you'd need the additional horsepower of 760sli/780 unless you plan to upgrade your resolution or use lightboost. If you do choose to go for a 500$ solution, you might as well just get a 780 IMO, i'd probably opt for that over 760 SLI but that's just me. It will be slower in some titles, but not all of them (depending on SLI scaling.)
The entire mining thing is just unfortunate. Prior to that, 7950s were easily found less than 200$ and 7970s were on sale for 270$ at times. At those prices, they were an incredible value - unfortunately those days are just long gone. Unless mining implodes again. Another thing I don't get - the 7950 doesn't have a replacement. It went EOL and the 280X is a 7970, while the 270X is a 7870. No 7950 replacement found. Not sure why.