Nvidia drivers include hidden overclocking software, requires a registry tweak to access them click here as always BACK UP the registry before making changes. If using XP make a new restore point as well.
To be honest most sources don't recommend more than 10% speed hike anyway and in most cases this isn't that noticeable. (any more and you're going to need some heavy duty cooling to stop it cooking itself ) click here . Unless you're a hard core gamer squeezing the last ounce from the card, the risks generally outweigh any benefits.