VPC 2007 cannot run 64bit guests (even when running on a 64bit host).  You
can run the 32bit versions of both Windows Server 2003 and Windows Server
2008 on VPC 2007.

Windows server operating systems are not supported guests on VPC 2007
however ("supported" means the right to get help from MS Product Support
Services for issues that arise).  If the availability of PSS support is a
requirement then you should use Virtual Server running on a supported host
to run a Windows Server 2003 guest.

Virtual Server also does not run 64bit guests.  Consider running Windows
Server 2008 as a host operating system if 64bit guests are a requirement and
running the guests under Hyper-V.