Ok download Rivatuner to OC, now go in small icrements like 5 mhz each time on mem. clock and core clock. also download templogger to monitor temp. (also use to set an alarm.)
Now after you go up a lil. you will need to test for artifacts.( trust me they are bad, signs of going to far.) you will have to download ATITOOL. It is simple to use also click on test for artifacts in the main menu.
Personally I will play my game for like 15 min after going up 10-15 MHZ just to see.
So when OC'ing a GPU go up lil by lil test for artifacts each time, and when you find artifacts just go down on mem. if that dont work put mem up a lil and go down on core until no artifacts.