Are you sure that both modules are DDR2? As far as I know, there's no 400MHz DDR2. If both are DDR2, then you're probably better off going for capacity than speed (have both installed).
Also, looking back in this thread, you've already asked this question and it's already been answered by patonb.
Also, looking back in this thread, you've already asked this question and it's already been answered by patonb.