I own a predator x34. I have also owned a 980ti, two 980tis in sli, a 1080, two 1080s in sli, and 1080ti whilst using this monitor. I can gauge the performance respectively with my experience. A single 980ti will not get the job done in most games. For example you will only get max 50 fps in GTA5 and average 40, on max settings. With two 980tis I was able to be way more successful closer in the 70 range. When I went with my 1080 FTW, its overclocked state did not beat out the two 980tis in sli but it was pretty close, and the new 1080 barely ate any power. But the two 1080s in sli hands down beat the 980tis and is ranked #1 in my performance metrics because together they were more powerful than my single 1080ti currently.
I don't remember playing a game I was not at 100fps with when I had two 1080s in sli.
You could easily get away with a 1080 and you would do fine in your games and with gsync to cover your butt things would be pretty smooth for any dipps in frames. However if you're willing to shell out a few more bucks (like 100 more?) for a 1080ti I would recommend this and it would be cheaper than 2 1080s. The 1080ti can get you very close to the target with noticeable improvement over the single 1080. If you're shelling out for the features of the x34 you might as well shell out for the GPU. With that said, I would stay away from SLI, I only ran into a few issues in my years of using SLI but with new cards nearly every year offering incremental improvements 10-20% you might as well constantly keep upgrading the GPU and selling the old one since (i hope) you are buying a monitor you will have for... 10 years (hopefully again).
And by only buying one you'll be less mad at yourself when new cards hit the market and you realize you just lost hundreds of dollars on your current GPU because used prices drop like crazy as soon as there is an announcement, even months before any preordering starts. (these announcements also make it the best time to buy GPUs used!, but given shortages and the miners who will want to get them for cheap maybe not this year..?)
TLDR Trust me, a 900 series card will not do it. Anything under a 1080 will not do it. Two 1080s is awesome but costs a lot. A single 1080ti is a good compromise on performance vs cost if you're wanting close to 100fps on ultra settings. (and you wont need a 40 dollar high bandwidth sli bridge).