The article I linked above describes it quite thorough. However you should have the voltage meter and good cooling ready if attempting this to stay in control 😉
I had my first 6600GT @ 1.7v core (instead of the stock 1.4v). This resulted in clockspeeds of 630/1330, with the card running at close to 80C under load, however.
Unfortunately, Gainward changed the card layout completely and I don't have a clue, where to Vmod my second card. Therefore I also removed the mod on the first one and cranked down again. Not the worst option for the health of the card I guess... 😉