When overclocking it is best to move the core up around 5mhz at a time until it becomes unstable when benched (furmark is a good one). Once you have idea how far you can go on stock voltage you can bump the core voltage up a bit and then move the core speed up some more. Memory doesn't seem to OC as well as the core most of the time, so I recommend leaving it as is. This can take some time, if you want, you can speed it up by googling 6770 overclocking to see what others are able to achieve. That will give you some idea of what you can get.