AMD was faced with a tradeoff during the development of the dual core Athlon 64 X2. In order to maintain backwards compatibility with earlier Socket-939 motherboards, they could not change the pinout of their dual core processors. While maintaining the same pinout resulted in the ability to upgrade virtually any Socket-939 platform to a dual core Athlon 64 X2, it meant that the dual core processors were left with no more memory bandwidth than their single core counterparts. The single-core Socket-939 Athlon 64s feature a 128-bit wide DDR memory controller, which when operating at DDR400 speeds, it gives the A64 a maximum of 6.4GB/s of memory bandwidth. Sharing the same memory controller, the dual core Athlon 64 X2s also feature the same 6.4GB/s of memory bandwidth, despite the fact that there are now twice as many cores vying for the same amount of memory bandwidth.

AnandTech