hwc GTX670 Review - Overclocking
Overclock the boost clock. As is stated in the article, the boost clock is the
maximum that the card will run at, not it's 'all the time' speed. There are a number of factors in whether the card will run in-game at the boost clock or not. Whether or not the TDP is being hit will determine whether the card will hit your full OC'd boost clock.
Sky managed an OC of 1259MHZ
boost clock on a reference card, but he mentioned that in-game the card didn't always run at that speed.