Overclocking, which is really easy with the i5-2500k.
You can overclock from 3.3ghz to 4.0ghz with almost no effort, and if you buy an aftermarket cooler for ~$30 you can go from 4.0ghz to 4.5ghz easily.
So for an extra $10 you go from 3.3 to 4.0ghz, and for an extra $40 you go from 3.3 to 4.5...