AMD have recently made building a new system around their processors a little complicated due to the compatibility, and moreso the backwards compatibility, of their processors and the motherboards that use the 3 very similar sockets for current AMD processors, socket AM2, socket AM2+ and socket AM3. Hopefully this guide will help to make things a little clearer.
SOCKET AM2 - This socket supports all AMD Athlon 64 CPU's (including single core and X2 models) and all Sempron CPU's EXCEPT the new Sempron 140. This was the first of the AMD Socket AMx designs. This socket can also support AMD Phenom processors but support is down to individual motherboards. If you have a socket AM2 motherboard and plan to use an AMD Phenom processor on it, you must check the motherboard manufacturer website to see if it is possible. The chances are that if your socket AM2 motherboard does support the Phenom series, it will still need a BIOS update before it can do so. Note also that if you install a Phenom CPU to a socket AM2 motherboard, the Phenom will perform a little slower than normal as this CPU is designed to use features and a Hyper-Transport speed that socket AM2 cannot support. With Socket AM2 no longer being produced, the chances are you will only have one of these sockets if you have an AMD PC or motherboard that is around 2 years old
SOCKET AM2+ - This socket was the first update to the original socket AM2 and supports ALL processors that are compatible with socket AM2, along with all AMD Phenom processors and the new Phenom II processors. You can even use the new socket AM3 processors on this socket. It could be said that if you are unsure then buying a Socket AM2+ motherboard is a fairly safe bet for your AMD processor. Unfortunately it is not so clear cut. Some AM2+ motherboards cannot support specific AM2+ Phenom processors, particularly the Phenom X4 9850 and 9950. This mostly affects socket AM2+ motherboards using the AMD 780 series chipset. It is essential that you check your motherboard manufacturer website for CPU support if you are using an AMD 780 series chipset motherboard. Like socket AM2, socket AM2+ motherboards use DDR2 memory only. They are not compatible with DDR3 memory. When using a socket AM2 processor on a socket AM2+ motherboard, the system will effectively slow down to the Hyper-Transport rate of the socket AM2 processor, it will not allow the CPU to run faster than designed even though the motherboard could theoretically allow it. When using a socket AM3 processor, the Hyper-Transport speed of the CPU is reduced, effectively resulting in the processor running slightly slower than intended, even though the clock speed (GHz) is the same. Neither of these situations are ideal, but are an acceptable compromise if you have a socket AM2 processor or DDR2 memory and do not wish to pay extra to replace one or both of those. Like socket AM2, no new socket AM2+ motherboards are being released, although unlike socket AM2, most major motherboard manufacturers are still releasing batches of their existing socket AM2+ motherboard line-up.
SOCKET AM3 - This is the latest incarnation of the socket AMx design and is also the strictest in terms of what it can support. This socket can only support socket AM3 processors, namely the specific socket AM3 Phenom II's, Athlon II's and the new Sempron 140. Remember that all other Socket AMx Semprons are Socket AM2 and the very first Phenom II's (most notably the 940 Black Edition) are socket AM2+, so are not supported by any socket AM3 motherboard. This socket introduces DDR3 RAM support and the fact that it uses DDR3 is the biggest reason why it can only support socket AM3 processors and offers no backwards compatibility for socket AM2 or AM2+ processors. If you are looking to buy a new AMD based PC and want DDR3 in your new system, this is your only option.
Hopefully this guide will make choosing the right processor, motherboard and even memory (RAM) for your new AMD PC easier.