The following quote is from the AMD website, so I would target getting an AM2+ M/B if I intended to drop an AM3 into it.
Although Socket AM3 processors can be used on Socket AM2+ motherboards
And this from Wikpedia.
Socket AM3 breaks compatibility with AM2/AM2+ processors due to a subtle change in key placement. The AM3 socket has 941 pin contacts while current AM3 processors have only 938 pins. Tom's Hardware removed the two obstructing key pins from an AM2+ Phenom processor in order to fit it into an AM3 socket. The processor did not work in the AM3 socket, but still worked in an AM2+ socket, suggesting that compatibility issues run deeper than merely the key pins.
However, AM3 processors are backwards-compatible with Socket AM2+, contingent upon a BIOS update for the motherboard. Manufacturers including Asus, Gigabyte, and others have labeled existing AM2/AM2+ boards as being "AM3 Ready" or similar, indicating that BIOS support is provided for the specified boards. This allows existing AM2/AM2+ systems to upgrade the CPU without having to upgrade any other components.