AMD has decided not to convert to DDR2 until 2006 when faster DDR2 memory will be available at todays 400MHs prices. The below excerpt is from PC World website.
"AMD's integrated memory controller design is much more sensitive to latency, or delays in a signal's journey from one point on a chip to another, Crank said. The first DDR2 chips run at 400 MHz and 533 MHz, and that's not fast enough to overcome the increased latency associated with DDR2 memory, Crank said. Internal tests of AMD systems with 400-MHz and 533-MHz DDR2 chips actually ran slower than systems with 400-MHz DDR chips, he said."