Yes you can save your iTunes and reload it after.
Yes Boot Camp comes with it, 2.0.
Yes you need to install Windows first in order to run it.
If it is an XP SP2 or greater disc, then yes. It must be a full version disc as well.
See this article for more details on Windows and Boot Camp:
Mac OS X 10.5: System requirements for Boot Camp and Microsoft Windows
If your system is < 32GB then yes. Otherwise no. You can just buy a 80GB or 120GB, 160GB, 200GB external disk and back up to it. You can then use it for Time Machine later (part of Leopard).
If you have the money, get the biggest disk you can afford. This will back up your system currently and also serve as a Time Machine backup drive. You can easily buy 500GB, 750GB, 1TB, etc as required.