I would just get the 2500K. As this article
shows, there is no appreciable improvement in gaming between that and the 2600/K. And the $100+ savings is substantial.
As far as overclocking...that's really up to you. The 2500K can overclock
from its max 3.7 turbo boost to 4.4GHz pretty easily. If you are worried about shortening the life of the CPU, I would recommend not
increasing the voltage. Leaving the voltage at stock value will limit your overclock, but it GREATLY reduces/elminates the negative side effects of overclocking. Also, remember that just because you overclock this CPU to 4.4 GHz doesnt mean it will run at that frequency constantly. It will likely spend a great deal of its time at the standard 3.3 GHz.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod