Generally speaking, there are two kinds of overclockers in this world. The first type consists of users who want to squeeze a bit of extra performance out of their parts. Sometimes the increase in frequency is modest, and other times they'll intentionally buy a lower priced (and lower clocked) part and OC the difference. Either way, it's a free performance upgrade if nothing goes wrong.
And then there are the insane folks at places like Kingpin Cooling, where LN2 flows freely and reference clockspeeds are, well, laughable. Consider the fact that well known overclocker Illya "TiN" Tsemenko just set a world record by overclocking an NVIDIA
GeForce GTX 690 graphics card to 1,547MHz. He had originally attempted 1,600MHz, but that proved unstable.
Image Source: TiN on Kingpin Cooling
At that speed, he was able to top 20,000 in 3DMark 11, posting a score of 20,962 using the Performance preset. That's pretty incredible, even if it's not practical because of the LN2 cooling required to scale that high.
After setting the world record, TiN turned his attention to overclocking a pair of GTX 690 cards in Quad-SLI, which he pushed to 1,458MHz for a score of 28,812 in 3DMark 11. And the scary thing is he's just getting started.
There are lots of pictures to drool over in the original thread