I had Vista Business on my macbook and ended up uninstalling it and going back to Windows XP. In parallels, you don't get to take advantage of any of the aero affects of vista and I just found all the extra vista stuff to peg my battery more than running XP did.
However, this isn't that big a deal for you, because its the mac tower.
In reference to the single use, if you install vista on your bootcamp drive, you need to activate it in parallels for it not to complain about activation. You can't activate in bootcamp first. I can see two options if don't see yourseld using parallels too much at first.
1. Install it anyway, activate windows in it, and just use bootcamp.
2. Hack vista to extend activation to 120 days+ until you might be ready to use parallels.