depends on what your games are ... but in General , you will be okay with a GTX 1080 and 3440x1440 , if you can pay for a 3440x1440 without a problem , I would suggest that you pay more and get the GTX 1080 ti and the better screen .
If you cant do that , still stay with the 3440x1440 and lower the details a little to get above 60 fps ... for those monitors put the ultra wide 1080p to shame
take a look here
http://www.toptengamer.com/gtx-1080-ti-vs-gtx-1080-ultrawide-gaming-benchmark/