It depends what you want to do with your PC
I have a i5 2500 myself and an upgrade to an i7 3770k (which I bought used) was totally worth it
I got my hands on a nearly unused gainward phantom gtx 770 really cheap price. At this time I played mainly Battlefield 4 with my former asus gtx 560 ti at medium to high settings. With the gtx I could improve the settings to high to ultra.
I monitored the usage of the cpu and when I played BF4 Multiplayer (which is known to be processor intensive) my cpy was most of the time at 100% usage. In other words: I could not use all of the possible performance of my gtx 770.
I upgraded to an i7 3770k an the load of the cpu went down to 60-70% in BF4 Multiplayer.
I think the i5 2500 was close at fully usingt the gtx 770 because the 3dmark scores are pretty close at fire strike:
i5 2500 + gtx 560 ti: 3252 points
i5 2500 + gtx 770: 6905 points
i7 3770 + gtx 770: 7367 points
A couple of weeks I bought a second gtx 770 for 120€ and use them in SLI mode and the i7 has no problem to handle it. The i5 would have been to weak at least in processor intensive games as mentioned. In addition I overclocked it to 4.3 ghz in turbo mode
i7 3770 + 2x gtx 770: 12042 points
conclusion:
1) yes, you can buy a graphiccard 970 and up and yes it will be an improvement, but you will not fully use its processing power due the bottleneck of your processor regarding to the games you play or programs you use
2) if you change your processor to a K and overclock you will see only a little improvement in games and a big in processor intensive tasks
3) if you get your hands on an i7 3770k and overclock it, you have enough power for gtx 980 ti in SLI mode so you can take the money you save on a new motherboard and price difference of a used 3770k to a new skylake k cpu (although thats not too much) and add it to your graphic cards budget
by the way, i use my i5 2500 again with a 750 as steam machine, which can do nearly any game running on steam os, still a good cpu