What are your Settings? I've got the Gigabyte 4GB but haven't been able to get more than 11 Mh/s, even with overclocking.
This is what I have them set at :
-dual=Decred -dcri 33 -tt 74 -fanmin 50 -fanmax 90 -mclock 2000 -mvddc 925 -cclock 1000 -cvddc 900