Boot Camp, Windows XP, and Parallels

Currently, I own a MacBook Pro with 512 MB of RAM and 80 GB Hard Drive. I
plan to upgrade these to 2 GB of RAM and a 120 GB Hard Drive. I have Windows
XP Professional installed on it under Boot Camp.

What I was wondering is:
I want to install Parallels [after I upgrade my computer's specs] and then
Windows Vista Home Premium or Ultimate. Since I already have the full version
of XP on it, can I use an upgrade version of Vista? Can I first transfer XP
to Parellels? I paid several hundred dollars for my XP and I dont want to pay
extra for a full version of Vista when Microsoft already has their extra
money from me for XP; also full versions of Vista are hard to come by since
not many people need them, so stores don't usually stock them.