lewis, every card will overclock differently, my powercolor 7850 hits 1190 core with 1.21v however yours may require more or less volts than this to run at a similar clock, all you need to do is increase your clock by 10mhz at a time and run the free heaven benchmark after you do, if your card completes this, then your overclock is 'probably' stable. if you fail, increase the voltage a bit (say 0.01-0.02v) and keep trying untill you fail it. once you have found a level you are happy with start overclocking the memory, if this fails you will see artifacting (basically dead pixels) if you see this, ramp down about 30mhz and leave it there.
NOTE: dont raise your volts above 1.225, the 7850 isnt safe above these levels
also, keep your temperatures below 85oC, however lower is preferable, personally i only hit 52oC
you may also find that after some gaming you crash or see artifacting, if so, then lower your memory to stop the artifacts or lower your core clock/raise your voltage to stabilize the core.
hope this helps, Lord tumnas
EDIT: to overclock use a program such as msi afterburner or asus GPU tweak, both are good, i however prefer GPU tweak, just because i do