Overall, not much if at all especially if you game with higher AA settings at 1080p or run a higher 1440p resolution. Both put more stress on the GPU vs. the CPU no matter if the game is CPU speed responsive. At best, a Skylake i5 6600 vs. your Ivy Bridge i5 3570 would only gain a few FPS with that 970. Other benchmarks, specifically productivity applications, are another matter though in CPU performance comparisons.
In short, you will run out of GPU power with that 970 before you run out of CPU power in gaming.