This from www.google.com, lots of info ,http://www.google.ca/search?q=Dual+Boot+between+XP+and+Vista&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
gets tricky when you want to use 2 separate drive ,as windows only has the ability to boot to one or the other ,you will at least need a boot manager program .google will help you find more info.
If you do change you mind and just use one drive ,2 partitions ,remember to load oldest, OS first .could cause driver replacement problems if done the other way ,
i have used this bootmanger , http://www.osloader.com/ .to dual boot xp, and 2000, on separate drives ,but i was using a scsi controller card and a scsi hard drive ,along with my master IDE drive .