When a single core is active, the chip can turbo up to 3.7GHz. If you want, you can change that turbo state to go as high as 4.1GHz. Overclocking these chips relies entirely on turbo however. In the case above, the fastest your chip will run is 4.1GHz but with only one core active. If you have four cores active the fastest your chip can run is 3.8GHz
the Core i5-2500K and Core i7-2600K are fully unlocked and will let you overclock them as far as the CPU and/or your cooling can sustain.