Get a new GPU
1070
For sake of more "future proofing" I'd say go with the 1070, if you insist on upgrading just to get a couple extra years out of it, you would also get away with not even replacing your GPU right now aswell. prioritise getting a good cooler and a decent OC, because you will notice more benefit from doing so, considering WoW is, always has been and probably always will be a CPU intensive game. Skipping the cooler and throwing a 1070 in your PC isn't going to net as much a benefit at that cooler and an OC will alone. at 1080 a R9 290 should suffice.
I wouldn't go near Xfire either, it's just horrid. Xfire and SLI are both a last resort in my mind, if no single card can do what you need, you get multiple, if a single card can, ALWAYS get that single card. You could get another 290, for not much less than a 1070, and that single 1070 will beat to 290's 50% of the time, and 10% of the time you'd be running only 1 card anyway because not everything supports it, somethings support it so badly that the second card causes a performance loss.. it's just so, so bad.
TL;DR - Get a watercooler and OC your CPU, upgrade in a couple more years.