I think you can safely go up to 1100 MHz as a starting point, then increase gradually and back off once you start to get errors, or graphical anomalies.
No 2 cards overclock the same so it is difficult to say. The best thing to do is increase it a bit a time and test it is still stable and not overheating.
I think you can safely go up to 1100 MHz as a starting point, then increase gradually and back off once you start to get errors, or graphical anomalies.