AMD Phenom II X2 550 Black Edition
The Phenom II X2 sounds really good on paper, being the same processor as the Phenom II X4 955 with just two cores at less than half the price.
Prior to the release of the Phenom II X2 550, $100 could get you either the Intel Pentium E6300 ($90) or the Intel Core 2 Duo E7400 ($120). The E7400 is an excellent processor, particularly at $120, but we can't help having serious doubts as to how well it can perform against a dual-core Phenom II processor clocked at 3.1GHz.
Looking at the specs, the Phenom II X2 550 would appear to have less L2 cache than the X4 processors because each core has its own dedicated 512KB. However because the L3 cache is shared the Phenom II X2 550 retains the full 6MB capacity matching the Phenom II X4 processors. The X2 550 also features the same integrated 128-bit wide memory controller supporting either DDR2 or DDR3 memory depending on the socket.
Furthermore, the Phenom II X2 550 is based on the same 45nm process with an approximate die size of 258 mm2 featuring roughly 758 million transistors. This processor has a thermal design power (TDP) of just 80 watts when working at between 0.850-1.425v which is 36% less than that of the Phenom II X4 955.
Provided your motherboard manufacturer releases the necessary BIOS updates, the X2 550 can work in both AM3 and AM2/AM2+ platforms. Given the similarity between processors this should be a non-issue really.