Is it possible,yes,But it's not easy!
It's best to install vista after you have XP already installed but it's still possible to do it the other way.
If the comp is new and you dont have alot of things you need to save,I'd wipe it and install xp and then vista,however there are a few utilities that will help you to install xp on a vista machine. I had a dual boot on this machine for sometime but found I could pretty much do everything on vista I could on xp so I just got rid of xp completely.
Other than being a resource hog vista isnt as bad as most make it out to be,sure it takes some extra work to make it run abit better but I've found it to be pretty stable for what I use my computer for. If your version of vista is 64bit you can use more memory than you can use with XP so load up extra memory and who cares if vista uses a gig or so!!
Of course YMMV.