The 2600k is highly superior for OC'ing, the 2600 does not have an unlocked core multiplier, whereas the 2600k does. If you're overclocking the 2600 you can play with the voltages, and it will turbo boost, but that's about it. Going for the 2600k you'll be able to OC to atleast 4.5-4.7Ghz, a few can go higher, and an even smaller minority 5Ghz+, but the k series is definitely worth it.
If you can't get the 2600k, get the 2500k vs the 2600. An OC'd 2500k will supersede a non 'k' 2600. Just make sure you look out for your temperatures, an aftermarket cooler will be needed, and ensure you have a nice power overhead.
[Addendum]: Also, if you're OCing make sure you have a P67 mobo, versus the cheaper, H67 mobos. Overclocking on the P67 is a lot easier, and a lot better.
[Addendum2]: I scan-read and thought that you were OCing, so yes, basically Beanoslim says it all. Although, if you do ever chose to OC, the small extra investment is worth it if there's a level of uncertainty.