To overclock your Gtx670 i would use evga's precision X, or msi afterburner. Note that nvidia has locked the voltage at 1.175v, and under load at stock it will normally reach 1.175v automatically. So there is no need to worry about messing with the voltage. The only way to unlock the voltage is through either a BIOS mod, or a hard mod, or by buying a card like the MSI Power edition that supports overvolting.
In general, without volt modding, 1300mhz on the core is a very good overclock, and its what most people hope to achieve.
Things to note while overclocking:
Set a custom fan profile to ensure the card stays under 70C as it will throttle by 13mhz at 70c, which will reduce your overclock slightly.
Sometimes it helps to max out the power offset, and manually lock the voltage at the maximum of 1.175..
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.