That is correct, you could use DDR400 in there, not DDR2-400. Which is a totally different voltage and pin configuration.
With Sonoma, you have an advantage of using DDR400 and higher vs older platform Centrino. As the bus will be able to take advantages of the faster memory.
The new chipset 915 allow the bus to be 533Mhz bus. Thus at
DDR-333 PC2700, it is running @ 333Mhz
DDR-400 PC3200, it is running @ 400Mhz
DDR-533 PC4200, it is running @ 533Mhz
So as you can see, faster DDR not only have better latency vs DDR-II for the time being, it will be able to be fully utilized to the potential due to the sonoma chipset.
So there is an arguementive factor that has been out there for awhile, DDR or DDR-II for sonoma.
Most places will say, DDR-II starts off where DDR left off. But that was when the introduction of DDR-II originally and DDR was at max PC3200. On the desktop side, the DDR have gone above DDR-533 and into DDR-600ish zone. So, with that, is it still better to use DDR-II anymore? In the desktop sense, not really, but DDR-II have room for move improvement.
Keep in mind that DDR-II is at a beginning stage. Just like every new technology, all beginning stages are always lack certain things and as time progress companies will be able to refine it more. Just like DDR when it was first released.