You will be able to buy a full
upgrade version of Win 7 (or 8), assuming your XP license is valid. The upgrade version is significantly cheaper than the Full Retail version.
The OEM license issue has changed with Windows 8 - MS now explicitly state that you can build a PC for personal use and use an OEM "Personal Use" license on that PC. However, this is for Win 8 ONLY, and you cannot use an OEM license for any previous version of Windows, nor downgrade the Win 8 OEM to Win 7.
See
Personal Use License
How thoroughly this can or will be policed is another matter!
SD