if you want the 250 to be your new XP
i would take the current hd out, plug up the new one install windows xp on it
then remove the new one from the machine…
plug the old one back up
then plug the new one up as a slave (both hds are now in the same system)
once you are up and running transfer all data from 20GB to 250GB that you need and place it in the folders that the fresh install of XP on the 250GB made…
once this is done…
unplug both hard drives from the system…plug the new one(250) as master, and the old one (20) as the slave.
fire up windows - make sure everything is kosher (i.e. you copied everything you need) then format the old(20GB) one.