gridironcpj
stalinx20
gridironcpj
If you can wait until January, then you could get a 1080 Ti. I would highly advise against a second 980 Ti at this time. I have two and multi-GPU support hasn't been worse than it is now. Of the recent games I've purchased:
- Gears of War 4: No multi-GPU support
- Forza Horizon 3: No multi-GPU support
- Civilization 6: Extremely poor multi-GPU support (although it doesn't really matter)
- Battlefield 1: Terrible SLI profile. I used a custom profile with the open beta, which worked much better. The same custom profile is no longer possible.
I purchased a handful of other games this year and multi-GPU support sucks for the most part. These are dark times for enthusiasts. I would also advise against a 1080, since it's not a large enough upgrade. The other option is to go for a Titan XP if you absolutely need to upgrade now.
And all the people saying SLI is "relevant"? pfff... Looks like it's not when you see the latest game(s) have no SLI support. 
Multi-GPU support has gotten much worse lately. The irony of a PC gaming boom is that the share of multi-GPU systems has decreased, yielding less "poops given" from Nvidia and developers. It was great a few years ago. Now, it's as if I only have one 980 Ti instead of two. I sort of NEED that extra horsepower at 3440x1440 for a lot of these games if I want to hit 100FPS.
Eidited that for poops given.
That's really why Gsync and Freesync is a thing, and it's changing the game. Do you find it odd that multi-GPU support is dying a slow, loud death because of a technology that makes it (mostly) irrelevant?
Granted, I find it inconvenient because I game on a 4K/60Hz 1080P/120Hz 65" TV, and prefer it to my spare PCs 27" 1080P/120Hz 3D vision monitor any day. And I don't expect Gsync or Freesync in a TV anytime in the next 5 years.