Monitor upgrade; 1440p 144hz V 4K 60hz

Right now I have an i5 4570, GTX 970, 8GB RAM on the Asus Z87K motherboard

I was thinking of upgrading monitor from 1080p 60hz to possibly 1440p 144hz or 4K 60fps. I've heard on few accounts that 144hz can cause issues with 60fps locked games? Also was wondering if I should upgrade to gtx 1080 for the possible monitor upgrade or wait for the next gen cards. Also I was thinking about my CPU, does it need upgrade to i7? Or do I need to look to build a new system with latest gen processor and RAM.

Also if it helps I'm in the UK