Oracle 10g express database link to Oracle 8i
Metalink 207303.1 is the Client/ Server Interoperability Matrix which will be the definitive documentation.  Since the express edition of 10g is based on Oracle 10.2, a database link to an 8.1.7 database should be supported.  Database links to earlier versions of 8i (8.1.5 or 8.1.6) are not supported from a 10.2 database.  For general sanity, I'd strongly suggest that the 8i side be running at least the 8.1.7.4 patchset but that isn't strictly required.