Tom's Hardware did a P4 overclocking test. they said voltage and cooling was VERY important. They say most 2.2Ghz will do 3Ghz, but require water cooling and 1.85v. Without water cooling the throttle protection would shut the system down as the temps went up.
He also said a distributor sent him a hand picked P4 that they had sorted to find (likely testing hundreds). This thing managed to hit 3Ghz with air cooling, but they said this was VERY rare. 3Ghz with water cooling and 1.85v was easy for most 2.2Ghz. He did have the radiator on his water cooling system outside his window so the system was running at 0'C.
He also stated that a DDR board was almost a must because the RDRAM could not take 133Mhz or more without dropping the speed to PC600 (indstead of the stock PC800) using a 3/4 multiplier. This was pretty hard on the performance, but the DDR memory could handle the higher speeds easily.
Now watch out which 2Ghz you get. Make sure it is the .13micron version with 512K L2 cache. If it is not you will likely not make it to 2.2Ghz. These are usually labelled as 2.0A or 2.2A for the new Northwood cores.
So a quick recap. DDR motherboard, good cooling, lots of voltage, a Northwood 2.0AGhz P4, and a bit of luck will give you 2.66Ghz without too much trouble (3Ghz if you go nuts).
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AthlonXP 1600+ @1.5Ghz
FOP32-1 Mod w/80mm (30db)
Epox 8KHA+ KT266A
256MB Crucial DDR266
GeForce2 GTS (210/410)
Sound Blaster Audigy
Antec SX-630 Case (mod'd)
TTGI TT-350-SS 370w PS
