Yes, upgrade
The 6.5 admin client will work against
a 9.01 server, but it won't have the many many changes in the admin client
that the 9.01 version will have. In general, the best practice is
to have the version of the admin server of the domain, match the version
of the design of the domino directory and admin4.nsf, as well as the admin
client. The reason being that new features/changes are often made that
affect all of those areas.