I've just been reading an article over at
Toms Hardware Guide where they have managed to overclock a P4 3.2Ghz to 5.2Ghz using liquid nitrogen.
CPU Speed: 5255Mhz
FSB: 309Mhz
Heat Sink temp: -196C
Chipset temp: -36C
Read the article here
Get the video here (DivX 5.1.1)
Very impressive stuff and worth the read