Ok, you're not going to see much improvement in streaming from a CPU upgrade from a 4790K..... perhaps additional RAM might be useful, but you'd need to monitor your usage (just in task manager would be fine).
Before you pursue a GPU upgrade, I'd recommend a better understanding on the hardware you currently have (specifically PSU and motherboard).
The PSU moreso, can play a large part in any upgrade. While a 1080 shouldn't draw a whole lot more power than your current 970 (180W vs 145W).... that 35W difference could be the difference between stable & crashes.