I agree with above, and stay away from MSI, their boards have a tendecy to blow up on their users randomly (barring the occasional success story and subsequent fanboy)
This is merely apocryphal, but over the past 2 years, I've built over a dozen systems with the MSI K7N series and haven't had one complaint. Most of these systems run in office environments, 40 hours per week.
I'm not a fanboy by any stretch, as I choose my builds based on research of market share versus number of customer complaints, compiling data from forums such as this one. In general, (not exclusive to S939) the most stable (read complaint-free)AMD boards are from most to least:
Tyan
Gigabyte
Epox
Abit
MSI
DFI
Asus
Now for some disclaimers:
1.This list contemplates neither overclocking ability nor overall performance. The Tyan boards do not even support overclocking at all.
2. Specific models/versions of boards may be better than others (note that although MSI is towards the bottom of the list, I still built over a dozen machines on the K7N boards, which were very reliable). I recall quite clearly the fiasco that was the Asus A7v when it first came out (which, by the way, pushed Asus' rating waaay down), and the subsequent revisions that were great boards. I see something similar happening today with the ASUS AN8 SLI boards.
3. I do try to take into account complaints that are based on user error, and understand that boards which are highly configurable (read: the best for overclocking) are also more suseptable to human error causing stability problems.
In closing, I would estimate that currently, the most stable 939 boards (note that Tyan does not make a 939 board; you'd have to go Opteron to use a Tyan) would be the Gigabyte GA-K8N series, the Epox9NDA series and an honorable mention to the Chaintech VNF4 series. Above disclaimers apply.