any gigabyte board with ACC, they are generally the best OCers.
785g chipset, with SB710, will not only be good for overclocking, but you may be able to make your 550 a B50 x4. Search "unlocking Phenom II cores" to learn more about it. I like Gigabyte, so I will link you one of them:
Since you have a BE chip your primary method of overclocking will be raising the unlocked multiplier. In that light any board that allows for vcore and multiplier adjustment would be just fine. It just depends on your budget but the motherboard JofaMang suggested is pretty good. It's even an Ultra Durable Board, though it's not ideal for crossfire since the second PCI-E slot is only 4x (rather than being able to have two 8x).
Best AM2+ board (IMO), has an SB750 (unlocks like a charm) and the price is not bad at all....