Intel told me from the day I got this processor 4GHz is possible on air and with water cooling 4.2GHz is possible. I've lost sleep trying to match what they said should be possible.
With water cooling the best I can pull off is this:

It was stable enough to benchmark and validate at 3.8GHz, but anything over 3.8GHz was totally unstable and the system simply turns off.
This is with watercooling, fans over the voltage regulators, on the open test bench with an OCZ PowerStream 620W PSU feeding the beast plenty of power. I've tried maxing out the voltage at 1.6V CPU and pushed the n/b and mem to their max with no luck of going higher. 20x200 will post and then blue screen on the XP Pro splash screen.
How are performance numbers at 19x200 on Dual Core? Well this is why I haven't done an overclocking article!
This is a STOCK Intel 840 Extreme Edition at 4-4-4-7 timings:

here it is at 3.8GHz:

Interesting eh?