I would actually suggest dual booting with Vista.. mostly because if you do a fresh install and format the drive, you could be out drivers that aren't reverse compatable. (ie: sony vaio laptops and most mobile laptops are specific to the OS preinstalled since the company doesn't publish drivers for other operating systems)..
worse comes to worse xp won't run in its entirety and you can just delete the partition in vista, or it works like a charm and you could delete vista..
Code:
http://apcmag.com/5023/dual_booting_xp_with_vista
^howto guide