I think your priority should be upgrading to an Intel processor. What are you using it for? If it's just gaming, an i7-6700k will give you slightly higher FPS than an i5-6600k, however it's not really worth the extra money, as you could use it to invest into a better GPU. If you want to do video editing or some sort of rendering, I'd highly recommend going with the i7-6700k. The hyper-threading helps immensely.
If it's only for gaming, what resolution and fps are you looking for, and at what settings (Ultra, High, Medium, Low?). Also, what games?