Cant boot at 400fsb = P5W DH.

[caater]

Does this mean that the P5WDH boots up with a 9x multi and then switches back to the user-setting?

yup.
p5w and p5b change multiplier by modifying exactly the same cpu register as, for example, crystalcpuid does.
first, post at default values and then bios applies new multiplier and boots up.
ES and XE chips are different - they are true unlocked cpus.
i totally missed that one from first post
indeed, to post at 7x400 you have to give cpu enough vcore like post at 9x400.