My answer still stands. You said that for your needs your pc is still running pretty good. So why start upgrading everything? (Unless I missed something, was lots to catch up on).
Get yourself a decent gpu, you got the choices that were listed, gtx1060/rx470 or rx480. This comes down to your willing budget to spend, you'll be happy with either choice, but I think it's between 1060 and rx480. If you will be thinking of keeping this gpu for next 3 years and play more AAA games coming years, take the higher vram cards, gtx-6gb or rx-8gb. I would take the 480-sapphire, or asus.
This cpu upgrade.. why?
http://www.tomshardware.com/answers/id-2952820/6600k-4790k.html
Why invest into old system? If you start doing more upgrades then just gpu and maybe ram, then you should plow right into the current gen. I know the 4790 or whatever was suggested is a nice cpu, nothing wrong with that, but again, building into the old.
As far as ram, just look for some 1866mhz+ ram kits. With price in mind, your motherboard is quite good, so it will handle higher speed ram. Don't need to go crazy and buy 3000mhz (example) but a bit faster will help (you have 1600mhz right now). Ram is starting to play a bigger roll in gaming, that's why I suggest a bit faster. You could explore options of buying used ram. Many people upgrading to ddr4 and they are selling their old parts.
I still think you should just upgrade what you need, and get a complete system when it's not performing well enough for you.
I'm pretty much on same page as Isokolon on this.
About windows. Right now, 7 is all that you need,. No reason to upgrade. In a couple of years you will want to get the win10 because a lot more games will utilize direct x12 (which runs under windows 10). Majority of games use dx11 for now, so really nothing to worry about right now.