Hello
The upgrade path you are thinking about is ok.
It should be possible to do a system copy from Ora 9 / Win2K => Ora 11 / Win2K8 and then perform the upgrade
=> 6.40 is supported on 2008 only during an upgrade.
1480906 - 6.40 SAPinst Patch for Windows Server 2008/2008R2/2012
The installation of a productive SAP NetWeaver '04 SR1 (Web AS 6.40 SR1) system is not supported on Windows Server 2008 or Windows Server 2008 R2 or Windows Server 2012. Only use the following procedure for a system copy or upgrade.
=> It is the same for Oracle 11
1478059 - Oracle 11g Support on Windows: 6.40-Based SAP Systems
Note: SAP Netweaver '04 SR1 and SAP R/3 Enterprise 4.7 based systems are supported on Windows Server 2008 (R2) only for upgrade purposes.
You might need to create a modified Inst Master media
1541738 - Creating a modified 6.40 Installation Master DVD
I've been through that process 2 years ago (but with a source release that was a bit more up to date => Win2K3 / Ora 10).
You might also need to perform a Unicode conversion, that can be done during the upgrade => check this note 928729 - Combined Upgrade & Unicode Conversion (CU&UC)
Regards