Hi,
Check out page 12 of the Compatibility Guide for SQL 2005 Express.
ftp://exftpp.symantec.com/pub/support/products/Exchange_Mailbox_Archiving_Unit/276547.pdf
Note that in Enterprise Vault 8 base is is not supported and pending for 8Sp1 and 8SP2, which means that it may eventually be decided that it will not be supported.
Previous versions are yes with important notes assiocated with them.
If you want my advice dont't do it.