As said already start HDToolbox, select partision drive, "activate" Advanced Options, select the partision that you want to change the priority on and on the middle buttom part of the screen you'll see "Boot Priority:" (partision has to be bootable or this is ghosted)... change the number there to change priority, the highest priority will run first.
But if the problem is that the computer boots of SCSI when you want the IDE to boot or other way around why not just unselect "bootable" from the partision and that should do it.
