Just found this information on how to install Vista fully from the upgrade dvd, this could be an even cheaper method for getting Vista as you only need to buy the upgrade dvd!
What you need is a copy of LegitCheckControl.DLL from any validated and activated copy of XP on a partition anywhere on your machine, then:
1.) create \windows\system32\ on any partition you want (even extended partitions)
2.) copy a validated LegitCheckControl.DLL file into the directory.
3.) you might need ntdetect.com and ntldr in the root, (try without)
http://forums.overclockers.co.uk/showthread.php?t=17685487
Another method:
1. Boot with the Windows Vista Upgrade DVD.
2. Click “Install Now.”
3. Do not enter a Product Key When prompted.
4. When prompted, select the Vista product edition that you do have.
6. Install Vista normally.
7. Once the install is complete, restart the DVD-based Setup from within Windows Vista. Perform an in-place upgrade. 8. Enter your Product Key when prompted.
http://www.windowsitpro.com/mobile/pda/Article.cfm?ArticleID=95011&News=1