I don't think a second 970 will be necessary for 1440p gaming. Go into your favourite games without VSYNC and see what your frames are like. If you're already getting 90/100+ fps then 1440p will still easily be stable 60fps. If you're only barely getting 60fps out of a game with a 970 (which, to be fair, probably means the game is badly written) try dropping settings down a bit. If that makes it go 90+fps then you'll be fine with those settings on 1440p, too. If dropping the settings doesn't give you 90+ fps then the game is a badly optimized piece of **** (*cough* like Arma *cough*) and switching to 1440p probably won't make things much worse than they already are.
If you're willing to spend £600 I'd recommend you keep your CPU and GPU and instead get a 1440p 144Hz GSYNC monitor like the Acer Predator or the ASUS ROG Swift (they both have IPS versions now and the acer is exactly £599). Especially for Arma, GSYNC is amazing. I tend to get 45fps on the average public mission (with my GPU barely even warming up. Thanks BIS!). With my old screen (also 1440p but locked to 60Hz) that looked choppy and weird. With GSYNC enabled (Acer Predator) it feels pretty much the same as 60fps (not 100% but 95% as good).
If you have games that your PC can drive to 100+ fps you'll also LOVE playing them. Fast shooters at 144Hz are super super fun but,and this really blew my mind: Normal desktop stuff (moving windows, scrolling text, etc) is AMAZING at 1440Hz.
In conclusion: Keep your 970 and 4.1Ghz i5 and instead go and take a dive into the wonderful world of 1440p 144Hz GSYNC gaming. It's a
much more noticeable change to your gameplay experience and you'll wonder how you ever put up with 60Hz non-gsync peasant screens