Disclaimer: Overclock at your own risk. Know your own limits, and never cross into unknown territory. When in doubt, back off on the overclock, and if you don't know what you are doing, forget about it.
I assume that you have DDR3 1600 Mhz memory or better (1866 Mhz / 2000 Mhz). I assume that your memory is XMP compatible (and no big deal if it isn't).
I assume that you have water cooling, or at least ultra high end air cooling (TRUE and above).
If not, don't follow my advice below. You would have to aim for a less ambitious overclock. I say again, do not attempt what I say below unless you have proper cooling. If not, abort mission till you get proper cooling.
Here goes:
Set bios to XMP mode.
Multiplier: 22 or Auto (assuming a Core i7 940 2.93 Ghz cpu; I personally use a 920)
BCLK: 175 Mhz
HyperThreading: On
Turbo: On (that will bump the multiplier to 23x)
Ultimate CPU speed in Windows: 4025 Mhz
CPU speed in Bios: 3850 Mhz
CPU Voltage: 1.35 volts (if possible) or 1.375 volts. Don't go past 1.375 volts without water cooling.
DRAM speed: 1400 MHz (175x8)
DRAM timing: 8-8-8-24-1T (it should set in XMP mode, except for the 1T). You can try for CAS7-7-7-20-1T, but its a long shot.
DRAM voltage: 1.64 volts (you will find that 1.65 volts defaults to 1.66 volts). Keep in mind that Intel says 1.65 volts and above *will kill* the *cpu* *and* *the memory* but that hasn't been my experience, but I'm not smarter than Intel
Bottom line: 4.2 Ghz is possible with some perserverance. 4.0 Ghz is relatively easy. Load temps (except in Intel burn test) in the mid 50s on all cores with 8 threads. Idle temps will depend on your room ambient temp and really is inconsequential.
***Don't even think of going above 3.6 Ghz without proper cooling.***
***Don't even think of going above 3.33 Ghz while using the intel stock heatsink*** (or you will see load temps 80 degrees and up.