Set your memory clock as high as you can get it without crashing, set your core clock at (memory clock * 0.605).
I tried setting this, 1375 *.605 = 831Mhz on the engine and my hashes dropped to about 420khash/s, re-upped to 1Ghz and it's hashing at somewhere between 480-500kh/s.
1375 is the stock memory clock. You need to raise it as high as it will go if you want good hash rates.
Some people are able to get up to 1800, but mine only goes to 1690 or so before it crashes.
Once you have stable overclocked memory, set the core clock with the .605 ratio.
These are the other settings that work well for me with that ratio: -I 13 -w 256 --thread-concurrency 8192 -g 2