Hi,
you won't have problems, as I did it myself on my P5W DH with both windows on a raid 1 and data on a raid 0.
Install xp x86 on partition 1 then vista x64 on partition 2.
If you install xp after vista you will lose the vista boot sector, but you can install it again by booting from the vista dvd and select "Repair my computer".
Brad