windows 2003 is essentially a build of windows XP for server use. I am rather confused about your orginization of Operating Systems. The operating systems should be grouped by simular problems.

therefore the most logical way would be:
Windows Vista/Longhorn(server)
Windows XP/2003
Windows NT/2000
Windows 9x/ME
Windows 3.11 and lower and DOS

it may be a bit of a stretch to pair NT with 2000, but it's alot less of a stress then putting 2003 with 2000..