You buy the 2007 license, but when you go to acknowledge and register the license on the eOpen website (https://eopen.microsoft.com , use Internet Explorer or you won't be happy) - it will provide the installation codes for both 2003 and 2007. As for installation media, I believe at this time they'll ship either just 2003 or both 2003 and 2007. If you only get 2007, you can call and of the
charity license providers found here and order the installation media for something like $20.