We have high hopes for overclocking today as the card is sold with a really good cooler and Sapphire are running the memory under specficiation, probably for stability and maximum longevity.
We managed to get another 140mhz out of the core and the memory was increased to 1350mhz from 1200mhz, giving a total effective throughput of 5400mhz (from 4800mhz). These are very impressive results for a HD5770 and while every card will be different these are good signs of significant headroom.