Definitely upgrade your CPU.
Your motherboard will have to be upgraded too, since the newest Intel processors are for the LGA 1151 socket type.
There is B150, H110, H170, and Z170 motherboards to choose from, but if you go with a K processor, pick a Z170 processor.
If you do upgrade, you will need to reactivate and/or reinstall Windows.
The GTX 970 is still a great card, so there's no need to upgrade at the moment, but consider it in the future.