Being rather tight with money, I would have to ask is your current PC not performing in a satisfactory manner?
A third gen i7 should still be a capable performer. Any CPU upgrade would necessitate a motherboard upgrade as well. If sticking with Intel, then the recent new Kabylakes should be considered; how much difference in performance you'd experience may vary on your games. Also... obligatory note about waiting to see what Ryzen is capable of as the release date is so close now. (If only to see if it does shake Intel's pricing.)
Perhaps a graphics card upgrade is more beneficial. Seeing you have a GTX 980ti it seems the next in line would be a GTX 1080 (or beyond). But even that may seem a bit touch and go for increased performance.