I really don't think you have to. The upgrade will blow out the partition and install fresh I'm pretty sure. I don't have that exact version here to test it out but from other posts on the Web it sure seems that way. At worst you'd have to insert your XP/Vista CD/DVD to verify and then it should move right along.
You can Google it and check on it more if you like. I just don't see paying an additional $100 on top of the $100 already saved to have a Full DVD.
