I followed this guide: http://www.techpowerup.com/reviews/ASUS/HD_7770_DirectCU/29.html
and overclocked my card close to the reference
Maximum stable clocks of our card are 1185 MHz core (6% overclock) and 1510 MHz Memory (31% overclock).
Great GPU overclocking. 1185 MHz is the highest clock we have seen on the six cards reviewed today. Unfortunately memory OC does not work so well. 1510 MHz is the lowest result seen today.
I pressume they oc'ed without touching the voltage. How much more can I gain if I raise my voltage? And how much should I maximum raise it to?