I think the max level I've overclocked the CPU has been 950 Mhz.
It's easy to do without void the warranty on the configuration set in the script raspi-config (called Turbo Mode), but there is a brave man who want blow up their Raspberry Pi (literally).
A forum user tells us under what conditions his machine has been overclocked to reach this processing speed. Want to see it?...
He tell us in the forum and did not use any active or passive heatsink. The configuration has been reached so far is:
He used PiBang, although I don't think is relevant. Do you encourage to do the same?...
...I knew you not.
Link: elinux.org > Overlocking