Another thing to consider with a cpu upgrade as far as cost and work involved, it's nearly a full system upgrade. You won't just be dropping a 7700k in place, it will require a new motherboard and ddr4 ram. At the moment ram prices aren't as cheap as they used to be. In addition to the $330-350 or so for a 7700k you're looking at around $140-150 or so for a motherboard to go with it. If you want to keep using 16gb of ram, modest ddr4-2400 2x8gb you're looking at $100-110. If you're currently using the stock cooler with the 4690k (since it's not oc'd) you'll want an aftermarket cooler for the 7700k (it doesn't come with one and a stock cooler from a previous gen may not be adequate).
That doesn't include the time spent to reinstall windows with a new motherboard. Not that it's the end of the world or impossible but the gains of going to a 7700k aren't just the cpu cost. You're looking at an os reinstall plus roughly $600 worth of upgrades just to use the 7700k and not short yourself on ram by downgrading from 16gb to 8gb.
You could spend around $340 and get a 4790k that would drop in place with your existing system, keep your motherboard, 16gb of ddr3 etc. It's not quite as fast but the cores are faster out of the box (500mhz faster) and cost of upgrade would be around half what you'd spend to upgrade to the 7700k. Just things to consider.
As others mentioned a gpu upgrade would be worthwhile especially if you're gaming across both monitors. It depends on how much of your gpu is currently used and if you're having to turn down graphics quality in the game to keep fps up higher.