Are you sure you don't want to touch the voltage? As far as I know, you need to flash your GPU's BIOS to unlock any unsafe voltage settings. If you want to leave the volts, slowly start raising the core frequency. To start, raise it by 10-25MHz, and use a program such as OCCT or ATI Tool to check for artifacts. I would let that run for only about 3-5 minutes. If no artifacts are seen or picked up, raise it another 10-25MHz, and run OCCT again. Keep doing this until you run into artifacting, or sometimes a lock-up. This means you are past your stable clock frequency on the core or shader, which I believe are locked together on these cards. Drop your core frequency by about 25MHz from where the clock frequency was set when you had artifacting (or lock-up), then run OCCT again, this time for about 20-30 minutes. If you aren't artifacting at the new clocks, you can fine tune another 5-10MHz higher if you want, testing in OCCT for 20 minutes each time, but it's not really needed. If you are showing artifacts, drop you core frequency by another 10-25 MHz and try OCCT again for 20-30 minutes. Once stable, there's your core clock. Remember that setting, or write it down if you want. Reset core clock to stock. Now, with core clock at stock, repeat all the above steps for the memory clock frequency. Once you have found a stable core clock as well as a stable memory clock, you can set them both and run OCCT one last time. That is a very in-depth method, I usually can find my cards max OC in about 10 minutes plus a 20 minute OCCT run.
Sorry for WOT.