AMD Athlon 64 x2 & Opteron dual-core processor review
 

The K8 has its own on-die North Bridge unlike Intel which uses the external FSB to communicate core to core. This on-die North Bridge gives plenty of benefits to the AMD K8 design for multi-threaded and multi-tasking processes. AMD have kept to their 939 & 940 sockets so that these dual-core processors are compatible with current motherboards which is great for everybody but means that it only uses the same memory bus which is still 128-bits wide and supports a maximum speed of DDR400 for 6.4GB/s.

http://www.a1-electronics.net/AMD_Se...64x2_Apr.shtml