If you're not worried about getting the lowest temps possible, a good cooler for chips that size would be the Zalman 7700. It definately fits the 939, and fits most cases (including small little mATX cases).
Should give you a good overclock with that.
If you want something bigger, the tT Big Typhoon is a good choice. That'll give you almost the best aircooling out there, with just a handful of choices which would be better.
If you really need the lowest temps, the Thermalright Ultra-120+ would be the best of the best. Even the Soniq Tower doesn't beat it (Soniq tower is still better than the tt Big Typhoon, and is considered equal to the Thermalright Ultra-120 [no+])
Of course, the tT BT would be just about the most sensible thing after the 7700, because the rest would require ATX cases, and you'd have to make sure that your m/b will fit it. Not to mention the cost, or the forces that'll put on your motherboard.
If anyone is thinking about the GeminII, that thing is good for scrap metal. Its too huge, and doesn't perform. Looks good though....