There ya go. So it is the same principle as pc 100 and pc 133 mixed memeory sticks. The Pc 133 will underclock to the PC100.
But why underclock the DDR 400? 1 gig is sufficient amount of memory and you might as well use the faster memory. By the way, two sticks of 512 PC3200 can be run dual channel as long as they are a matched pair.