>With XP, there was XP and XP Pro. With Windows 7, AFAIK, there is only one version. So, should be just put the CD in it and install on top of XP Pro?

There're more than one edition of Windows 7 - http://en.wikipedia.org/wiki/Windows_7_editions.
Windows 7 installation will wipe out your WinXP. There's no in-place upgrade.