1. I've never done an upgrade from SQL 2000 to 2008, so I'm going to leave that one alone.

2. I looked up the requirements for SQL 2008 and you're right - it does say Windows 2003 SP2 or above. I would definitely listen to this. Microsoft probably knows things about their operating system that we don't, so I would be safe rather than being sorry later.

3. As for backing up a database, moving it to another server and restoring it there - that's exactly what I would do. In fact, that's what I do when I have to copy databases from production to test. I think you'll find that this is a preferred method of copying databases from one server to another.

For the exact requirements: http://msdn.microsoft.com/en-us/library/ms143506%28v=SQL.100%29.aspx