http://www.anandtech.com/memory/showdoc.html?i=1977

Quote from article:

What does this mean for early adopters? Essentially; nothing. There will not be a performance increase between DDR2-400 and DDR-400 (or even between DDR2-533 and DDR-533). DDR2 is the technology to enable post-DDR533 speeds, rather than a technology to enhance it. As we start to see benchmarks of DDR1 versus DDR2 trickle in, consider the maturity of the two technologies. DDR1 is nearly 5 years old with dozens of chipset manufacturers and billions of dollars in financial backing. DDR2 still has not even entered full production yet.
This says it all.