You would have to play a bit with the setting to find what values will be stable for your card. You can try to set memory Clock to 200, 300, 400, ... all the way to 1000 to find a value that works for you. Higher the better, but too high will make your miner to crash. I'm using 450 on one type of card and 850 on different card. For core clock 100-150 worked for me.