theres lots of ways,..but ill say this first,..if your not familiar(im guessing your not?) be cautious,..seriously,..
you can get coolbits which will hack open the settings locked in nvidia drivers,...yes nvidia has an overclocking utility built into the drivers but they lock it for obvious reasons,...along with refresh rate overides(to lock th erefresh rate in games the same as it is on the desktop) and some other settings as well
once coolbits unlocks it there is a tab that says "detect optimal settings" it will work your cards temp to find teh safest oc,...on some cards it under shoots,..and on some it has overshot(rarely but ive seen it),..if you wanna do it manually,..just raise it 5 mhz at a time on the core and memory test it to see if it will take and play a game er somethin to break it in then rinse and repeat,....but be cautious and dont go to far,..if you see anomolies in th egraphics or artifacts stop,...if additional hitching occurs,..stop if it locks up(obviously you stopped

) if anything wired happens then stop and go back in and back off the clocks about 5-10mhz and try again until its stable,..just use yer head and you will be fine
ForceWare Coolbits 2.0