It's my understanding that XP wants to be only on the primary hard drive, so booting from an external will be difficult, if not impossible. You *could* install OS X on the external, and XP on the internal, but why would you want to do something stupid like that ?
So you have both XP and OS X on the internal, and would like to store data on the external? I think this is straightforward. Can't XP see the external drive when plugged in using FW?
How is the drive formatted? If it's formatted NTFS (typical for Mac OS) then Windows can't see it. FAT32-formatted drives can be seen by both. There is more information on this on the web, I'll look for a link (try macosxhints.com).