The question is, what resolution do you drive?
If it is FullHD, i think your GPU is in fact the bottleneck, and the CPU won't have any noticable effect (i have the same GPU, and recently updated from a 3GHz i5-3550S to a i7-3770 - makes no difference at all).
I just saw an article pointing to the 980ti - buying that one in used condition would provide okayish results on 1440 (which the 970 would not be able to handle), and very good results on HullHD (still definitely better than the 970).
Go to ebay, and check prizes by checking "sold articles" (or the like) to see what people really pay. I see an average of €350, which is about €100 better than a 1070, and still better than the 1060, which is slower than the 980ti. Don't check for "new", though...
If €350 is a good investion into a used card is an entirely separate question, though...
These are difficult times for gamers....