Opterons are generally intended for stable server use. You will not find too many system boards that lend themsleves well to overclocking. It is possible though.
As for which board to use, it is best to define what you intend to do with said system. If you want cheap, you can get an MSI or tyan board. No PCI-X, but AGP, RAID, SATA, up to 4GB REG ECC.
With all of the opteron boards you will need Registered ECC.