Open afterburner and incrementally increase the core clock (maybe 25-50 Mhz at a time). Test for stability/temperature after each incremement and see how high you can go. If it gets too warm force the fan to run faster in afterburner. I use kombustor and let it run for at least a burn in test between steps. When I think I find a good stopping point I will run kombustor for an hour or so. I also keep a log of benchmark results so I can see the difference the OC makes.
my card runs at 1.190 volts and i would feel safe up to the voltage limit for it, which is 1.225 volts, however i would keep an eye on temps, after about 90oC the card will begin to throttle itself down.
you should also be able to reach 1400mhz on your memory without artifacting or memory checking reducing your performance.