"The Accelero Twin Turbo was the winner in every test. The factory heatsink was clearly outmatched by a far more superior product. The temperature differences between the two products were as high as 36 degrees in some tests. At its best, the factory heatsink got within 6 degrees of the Twin Turbo. In addition, the Twin Turbo fans were much quieter than the factory fans at every speed setting. At any speed above 50%, the factory fan was simply unbearable. AIso, I decided not to test the stock heatsink at an overclocked load with only 25% fan speed, in fear of damaging the card.

http://www.overclockersclub.com/reviews/twin_turbo/