If you were wanting 4.2ghz on your 2600, a good air cooler should do nicely.
I'm using the Be Quiet! Dark Rock 4 Cooler(200watt TDP, Single Fan) on my 2600 at 4.0ghz with ~1.34V and it keeps the CPU quite cool. Highest sustained temps I've seen so far are around 42-43c, and that was playing WoW in an 16+man raid with lots of effects and on screen action going on for about 2 and half hours. The 2600 isn't really a hot chip to begin with which is nice.
That being said, you'll want to make sure your case has good air flow so that the cooler can be fed with the air it needs and the hot air can be exhausted.