Looking for some more information about the gradual upgrade from SPS 2003 to MOSS 2007 I stumbled upon this monster step-by-step guide of Cornelius J van Dyk, a Microsoft MVP in SharePoint. If you are starting an upgrade project, then be sure to go through this document!
You can find the document and the post at http://www.cjvandyk.com/blog/Lists/Posts/Post.aspx?ID=107
And be sure to add Cornelius’ Blog to your favourite feeds!
And if for some weird reason you are reading this Cornelius, GREAT JOB on your blog! You just got an additional fan!