MSI K8N Neo4 Platinum, or its smaller brother, MSI K8N Neo4-F... Although not SLI (but that's a feature I wouldn't spend money on IMHO... I just don't like the fact that a single 7800GTX can outperform a SLI setup with 2 6800GT's worth $ 800 most of the time..), they offer nice overclocking capabilities (mine went to 331 "FSB" with a 3000+ Venice) and have a lot of features...
I don't have personal experience with them, but I'd say that AsRock probably isn't very overclocker friendly (maybe that's changed now, I don't know!), but I always thought their aim was just to make mainboards that are stable, and make them as cheap as possible (a bit like ECS). 
I've had GigaByte mainboards (although not for Socket 939), and I was very happy with them, they always proved to be very stable... so I'd consider GigaByte too...
Other brands that you'll certainly be happy with (for overclocking): DFI, Abit, EPoX, ASUS... if you pick one of those, you shouldn't run into troubles... Just stay away from the K8T890 mainboards... Their feature set isn't that rich, their overclocking capacities aren't as good, and performance is mostly below the one of nForce4...