Gutsy : how to change the default in Power Management>"Put computer to sleep when inactive for" to hibernate?

[Loïc Martin]

Hello,
Since my computer fails to sleep (even without Desktop Effects) but 
hibernates ok, I'd like to change the default policy from Sleep to 
Hibernate after a period of inactivity (else I'm force to never allow it 
  to save power by itself, which is quite a drag).

Thanks a lot,
Loïc