Yep, leave the voltage the same, and start by increasing the core clock speed by 10mhz, and then run a benchmark like unigine heaven. If it goes well (no crashes), increase the core clock by another 10mhz, and repeat until you are no longer stable. Then go back to your last stable setting, and increase the clock by 2mhz until you reach your maximum stable clock. Also make sure you keep your temperatures below 70C so the card doesn't throttle itself by 13mhz, however if it does then its no big deal, you just lose 13mhz, just keep the temperatures under 80C overall. Once you have reached a safe stable overclock on the core, report back and if you want you can OC the memory too. However overclocking the core yields more performance per mhz, than memory. But you can OC both.
So just start with the core, and watch the temps.