If you try and join a new server to the farm, but can't get the config wizard to load the list of databases from the SQL Server, double check the firewall settings on the SQL Server and SharePoint server.  You will often see what seems like a really bizarre error; something like System.ArgumentNullException was thrown.  Additional exception information:  The errorData argument cannot be null or zero length.  Parameter name:  errorData. Chances are the exceptions for SQL Server have not been properly configured.  This has already tripped up a few different people I know, and since SharePoint is now going to require Windows Server 2008 you may not be used to having that firewall in place.  Just one quick thing to check and try before that first wave of installation panic sets in.