i7-4790K, even if you can't OC it with your MoBo, is solid CPU and will do just fine for years to come.
Here's also comparison between i7-4790K and i7-7700K,
link:
http://cpu.userbenchmark.com/Compare/Intel-Core-i7-4790K-vs-Intel-Core-i7-7700K/2384vs3647
As you can see, even the best Kaby Lake CPU isn't a lot better than the best Haswell CPU. Though, the best Coffee Lake CPU (i7-8700K) is better since it has more cores and threads than previous generations i7 K-series CPUs.
In my Haswell build, i have i5-4590 and at current date, i can game @ 1080p with high/ultra settings just fine (full specs with pics in my sig).
Though, if you want to improve your gaming performance, the biggest gain is from new GPU, e.g GTX 1070. While GTX 1060 is best GPU used at 1080p, it isn't much better from GTX 970,
comparison:
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-970-vs-Nvidia-GTX-1060-6GB/2577vs3639
For GPU upgrade to be worth the money and performance gain, i suggest looking towards GTX 1070,
comparison:
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-970-vs-Nvidia-GTX-1070/2577vs3609
pcpp:
https://de.pcpartpicker.com/products/video-card/#c=369&sort=price&page=1
While GTX 1070 is best used @ 1440p, you can game @ 1080p without issues. Since GTX 1070 is more powerful than GTX 970, it will last considerably longer before upgrade is due, even if you play @ 1080p.
Here's also a brand comparison if you don't know which GTX 1070 to go for,
link:
https://gpunerd.com/guides/best-gtx-1070
For a better decision making, here's a 2 builds comparison, i7-4790K with GTX 970 as a baseline and i5-4460 with GTX 1070 as an alternative (since i don't know which SSD you have, i put in the most common one),
Userbenchmark PC Build Comparison
Baseline Bench: Game 65%, Desk 86%, Work 63%
CPU:
Intel Core i7-4790K
GPU:
Nvidia GTX 970
SSD:
Samsung 850 Evo 250GB
HDD:
Toshiba P300 1TB
RAM:
HyperX Fury DDR3 1600 C10 2x8GB
Alternative Bench: Game 89%, Desk 67%, Work 46%
CPU:
Intel Core i5-4460
GPU:
Nvidia GTX 1070
SSD:
Samsung 850 Evo 250GB
HDD:
Toshiba P300 1TB
RAM:
HyperX Fury DDR3 1600 C10 2x8GB
As seen above, better CPU gives you better workstation performance (e.g video rendering) and also better performance in desktop usage (e.g web browsing) while better GPU gives you better gaming performance, even if you keep your current CPU. While upgrading both (CPU and GPU) would be ideal, it also will be way out of your budget range (about €800 in total). So, if you need to pick either CPU or GPU, go for GPU. You can also sell your GTX 970 and get some of your money back or keep it as a backup GPU.