Good choice moving to s939!
The best mobo chipsets right now on the 939 platform are nF3, nF4, and RX200 (and the other Radeon Xpress chippy, can't remember the number).
If you're looking to stay within a pretty tight budget my suggestion for best overclocking mobo would be the DFI LanParty Ultra-D. If you're looking to eventually upgrade to an SLI setup I also highly recommend the EPoX 9NPA+ SLI.