Hi, for sure there is a firmware available for the Cube :

http://support.apple.com/kb/DL1285

Leopard Assist works pretty well, but you will probably need an original Leopard Disk to get it running.

tried several times with a bad copy, ending up with a KP !!... worked like a charm with the original DVD (several Powermac G4)

So far haven't tried in the Cube, Tiger should be more than enough (Leopard is quite slow on a 450 Mhz G4)