Today when I needed to upgrade my VC database from MSSQL2000 to MSSQL 2005 i came upon these quick guides.

http://blog.core-it.com.au/?p=27

http://spininfo.homelinux.com/news/VirtualCenter/2008/01/06/Howto_Move_Migrate_Design_VC_DB_from_SQL2000_to_SQL2005__