If you get an OEM copy, the license is only good for the first machine you install it on and that machine only. Microsoft will not let you install it on a different machine if you change your computer, even if you've removed it from the original machine.
If you buy a retail copy, Microsoft will let you move the OS from machine to machine, although it may only be installed on one machine at a time. The software can also be legally sold/transferred once from the original owner to a second owner.
Since an XP installation requires activation through Microsoft, I think it's a good idea to buy a retail copy since the license is more flexible.
If you decide to buy XP, make sure it's XP w/SP2 integrated (i.e. on a single disc).