AM2 Soldiers On
Socket AM2 motherboards and processors first arrived at retailers back in 2006, making the socket about three years old. Normally, this would mean that the socket is at the end of its life-span - for comparison, Intel effectively announced the imminent obsolescence of LGA 775 late last year when it debuted Core i7 as an LGA 1366 processor. But AM2 continues on. There is little reason to believe that AMD will be quick to abandon its Athlon X2 processors, which remain entrenched as good values in the budget market, and AMD has also continued to provide those with AM2 motherboards with upgrade options.
Officially, AM2 is completely compatible with AM2+. However, there are unofficial difficulties that are common if you're attempting to use a motherboard from the dawn of the AM2 socket with a new AM2+ processor. The first major obstacle will be BIOS compatibility. Obviously, a company can not program their BIOS to recognize processors that will show up three years later, so the stock BIOS on older AM2 motherboards is not going to recognize a new AM2+ processor. You'll need to make sure your motherboard manufacturer has provided an updated BIOS with support for new processors.
More difficult to tackle can be the problem of power. New, high-clock, quad-core AM2+ processors draw much more power than old dual-core AM2 processors, and some old AM2 motherboards can't provide the extra juice. This is a difficult problem to nail down, as both AMD and motherboard manufacturers are fairly silent on the issue. Some motherboard manufacturers do provide updated compatibility charts on their websites, however, so visiting the manufacturer's website is a good place to start your investigation.
What about AM3? Surprisingly, AM2 does theoretically support AM3 processors. There is no inherent socket incompatibility, and Gigabyte lists some of its old AM2 motherboards as AM3 ready. There is, however, some red fine print that defines how the AM3 processor will be limited in the AM2 socket.