The domain name change is totally separate from the other upgrades. After the domain has been renamed the SQL Servers will need to be rebooted. Nothing else should need to be done after this. You can optionally change the names of the Logins and users to match the new domain name. You don’t have to however.
As for the system and SQL Server upgrade, if you are going to be upgrading the existing server, I would recommend that you backup the database and simply format the server. Changing the OS that SQL Server is running on via an upgrade is just asking for trouble. Especially as you plan on upgrading SQL Server after that.
Do a clean install of Windows 2008, then install SQL 2005 or SQL 2008, then restore (or attach) the databases to the new instance.