restoring a database from a file copy

Can a database system be restored just from the data under the 
installation directory?

I backed-up my database using "rsync /var/lib/pgsql "

Then I upgraded my rpms from 7.3.4 to 7.4. Then I tried to start the 
server as usual and it complained about version conflict (of course).

I removed the 7.4 rpms and went back to 7.3.4 and restared server.

My data are gone - only the template tables are listed.

I copied back from my file backup and this, much to my horror, did not 
restore my database - it's as if the tables are all gone, but the file 
sizes indicate the data are there.

Any way to recreate a catalog (or whatever) from the data?

The server seemed to be down when I copied - can't see what I could have 
done to corrupt the files under /var/lib/pgsql/data/

I couldn't do a pg_dump due to bad OIDs - this was the motivation for 
the upgrade - so I seem to have lost everything.

Any thoughts?

-- 
Chris Fjell