If you choose to purchase an upgrade version of Windows Vista to upgrade XP, you will no longer be able to use that version of XP. Either on another system, or as a dual-boot option. The key will be invalidated, preventing activation.
From Vista’s EULA found here (PDF) :
13. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligiblefor the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from.
For many people this may not be a problem, but it’s a change from earlier versions of Windows.
Update: Workaround posted here!