I would upgrade to 5.6 then to 6.0.  I am working late again because I updated 5.5 straight to 6.0 and my server died.  Domains got lost, mail got lost, lost connection to internet....etc.  Good thing I had a backup.  So I am stuck here, reloading 5.5 with all the updates to get the server to the state that it was in so I can restore the backup from workstation, which I may add, has saved my butt.  Thanks Darrell!  To upgrade to 5.6 then to 6.0, so I dont have any broken components in my server.
HTH