Run it at stock speeds for a while.  I would also set intensity to 19.  Once it is stable at stock speeds then you can start playing around with bumping things up, although with scrypt mining it's more about ratios than how far you can push everything.

You can also look at recommendations for 5800 series cards from this thread - https://bitcointalk.org/index.php?topic=117221.0

good luck (again).