I've been using an i5-2500K @4.5ghz for the last five years running FSX, FSX:SE, P3D v3, XP10 and now 11. I started with a cheap GTX 460 and then upgraded to a GTX 960 as well as using a 29" ultrawide monitor now. This system/setup has been decent with most payware scenery and payware GA aircraft, but I've never been able to use the NGX or other fps heavy addons. I'm always hesitant to upgrade because I know the newest and best is always about to come out, so I just wait things out with my trusty 2500K. 

However, I'd finally like to be able to move up and fly larger, more advanced aircraft and be able to fly into payware airports. I tried using the XP11 default 737 with the Zibo mods last night in overcast conditions and was only around 20fps most of the time with stutters which made for a disappointing experience. 

I was thinking of going with a 7700K and GTX 1070 (1080 is too expensive for me). Will this allow me to use the NGX, JAR A320, etc?

I'm also hesitant to purchase with P3D v4 on the horizon, but it doesn't look like there will be any major advance in hardware in the near future. I'm tempted as well to just upgrade the GPU and see how a 1070 will perform with a 2500K. 

Thoughts?

I'm nearly on the same boat as you

got a GTX 970 myself 

I think you'd be ok with Z270 motherboard, 16 gb ram and a 7700K, and gtx 1070

Took the plunge Ryan and ordered with what you  are thinking of for X plane 11. 

Should be fine, though you haven't stated the resolution of your monitor.  I do fly with a 4k monitor and a 1070 and it does alright but I've seen FPS get down into the 20s in my worst-case test scenario flying over the terminal @ JFK looking out towards Manhattan in the NGX.  Despite the resolution though my guess is that I'm still CPU-limited in that scenario, even @ 5.2GHz.