The x34 is 3440x1440, so it is going to require more GPU power than a regular 2560x1440 monitor since it has about 25% more pixels to push. The GTX 1080 is roughly equal to 980 SLI (+/- depending on the game). In all benches I've seen the 1080 get at or above 80FPS at (regular) 1440p.
However, keep in mind some games do not scale as well in SLI (poorly supported), so the single 1080 will be better in those games, meaning you may see dips below 60FPS. It's really hard to tell since I've never seen that resolution tested with various GPUs and FPS comparisons.
EDIT: I noticed some are recommending a single 1070. That is not going to suffice since the heavier frame rate hitting games (like Witcher 3, Far Cry 4) with the 1070 barely get above 60FPS at regular 2560x1440...remember your resolution has 25% more pixels than standard 1440p.