I would not upgrade from a 2500k to a 6600K. If you are going to upgrade, I would go for more threads, especially with DX12 games coming. So I would go to either the 6700k or the 5820k. In either case, you will have to purchase a new motherboard and ram as well as the cpu, so the extra hundred dollars or so for 6700k or 5820k is not a huge additional cost. Between those 2, it is a tough choice. 6700k has faster stock clocks and better ipc, while the 5820k(or BW-E) has more real cores but you are depending on a good overclock.
With a 2500k at 4.5 though, I would be tempted to wait and see how it performs with the new gpu and in DX12. You might even hold out to see if Zen is a viable alternative, although it wont be available till late 2016 or early 2017.