I believe you are correct on both accounts. MG 6 is not supported on Windows 2003. Although it may run on Windows 2003 SP1. MG 6.5 SP1 was just released and it runs on Windows 2003 SP1.
I believe you may need a new license to upgrade to 6.5. You would have to clarify this with your Autodesk vendor you purchased MG 6 from.
Regards,
Dave