The GTX 1060 would be a good upgrade. As far as the CPU holding back the GTX 1060 I doubt you'll have a issue but it depends on the game and is still going to be a massive improvement over that 660. Get the 1060 then test it during gaming, if adjusting the graphic settings doesn't change your FPS then your CPU is the issue but IMO I doubt you'll have this problem in most games if your gaming at 1080P/60 FPS.
https://davescomputertips.com/how-to-determine-gpu-vs-cpu-bottlenecks-and-possible-solutions/