For my tuppence worth, the monitor is one of the most important upgrades/purchases for your system. Everything else, is only their to put pixels on your screen fast enough to provide a good gaming experience.
4K is great, but even the 1080GTX struggles to drive this resolution at anything but "acceptable" frame rates - unless you have the cash to go SLI.
3440x1440p is somewhat of a sweet spot here - playable frame rates, increased immersion from the widescreen and high resolution without dragging your GPU to its knees.
Yes, £900 for an Asus X34A is a big stack of cash - but look at what the rest of your system costs (Nevermind the £600+ for a 1080GTX!) - the monitor is the end point of all that expense, so don't skimp on that one component.
If you are happy at 1080P, great - but go for one of the dropped cost last-gen NVidias - at most a 1070. If you want all the glory that the new 1080GTX brings, get a decent 3440x1440P screen with G-Sync so you can appreciate what you have! G-Synce comes into its own as you start dropping below 60fps from what I understand - the benefit is negligible as you move to higher playable refresh rates vs frame rates.
Like I said, just my tuppence worth. Everyone has their own opinion and sweet spot. Its your money, you need to decide what you want from it!