there isn't a matrix -theres should be one but for a long time we've supported virtually every permutation, so normally it was "everything except sql 7"
a quick one off the top of my head:
supported database: sql 2000, msde2000, sql 2005, sql 2005 express, sql 2008 , sql 2008 express [other databases such as oracle are available via 3rd parties] . Note: there are issues with dotnetnuke & 3rd party modules, so you're recommended to use non case-sensitive collations such as the default.
supported OS: windows 2000, windows xp, windows 2003, windows vista, windows 2008
supported webserver: IIS 5, IIS 5.1, IIS 6, IIS 7 (both classic and integrated)
Note: with 5.0 you need to have MS Ajax installed.
Cathal