Few games use more than 2 or 3 cores. The extra hyperthreads of the 2600K will go largely unused. That is why the 2500K, or 3570K at $100 less is the better option for gamers. Either cpu will OC to about the same level as the non HT versions.
The GTX670 sems to be a much better buy today than the GTX68o which costs $100 more.
How high you can OC is determined by your particular chip sample, and the quality of your cpu cooler.
A 2500K @4.5 is not unreasonable, but it is not certain.
Similarly a 3570K @4.3 is also reasonable. Why buy the 3570K? It is because the ivy bridge architecture makes it about 5% faster, clock for clock.
For gaming, once you get past 4.0 you can run anything well, and the graphics card will be your limiting factor.
My suggestion is to save $100 and gat a 3570K.
Save another $100 and get a EVGA GTX670FTW.
Use the $200 saved on a SSD for the os and some games.