The first point to address is exactly what differentiates a Pentium 4 Northwood from the older Willamette versions. In a basic sense, any Pentium 4 at 2.2 GHz or above is a Northwood, and Intel has also taken the core back down to the Pentium 4-1.6A, 1.8A and 2.0A models as well. There are no Northwood models available in the 1.3, 1.4, 1.5, 1.7 or 1.9 GHz speeds, and these Pentium 4 models all use the older core technology. The easiest way to determine which processor you're dealing with is to find out how much L2 cache the processor in question has. The older Willamette has 256K and the Northwood ups that to 512K, and this is the true litmus test in identifying the actual Pentium 4 core.
There are a number of factors present in the choice of the optimum Pentium 4 overclocking CPU. To illustrate the case better, here is a chart with the three main contenders, along with their multipliers, default FSB speeds and the potential core speed if taken to the 133/533 MHz bus:
Speed at 133/533 FSB
After viewing this chart, let's use a bit of logic in determining the optimum buying point for overclockers. The Northwood core was really introduced with the Pentium 4-2.2 GHz processor, and given that all Northwood models (1.6A to 2.53 GHz) currently use the B0 core revision, the minimum speed you should expect would be 2.2 GHz. This logical hypothesis also gives a better idea on why the Pentium 4-1.6A is such a popular choice, as its 2.13 GHz overclock using the 133/533 FSB is still below the 2.2 GHz magic number.
Now this doesn't mean that many Pentium 4 Northwood processors won't achieve far greater overclocks, but it simply spells out the safest bet. The highest speed Pentium 4 is currently the 2.53 GHz, which can also give us an estimated ceiling for high-end overclock speeds. The Pentium 4-1.8A is growing in popularity, given that the median for Pentium 4 Northwood overclock levels seems to be in the 2.4-2.5 GHz range, or very close to the high-end of the standard Pentium 4-2.53 GHz. This also makes the Pentium 4-2.0A GHz a lot more risky option, and although some have taken their processors to the 2.67 GHz level, the success rates are naturally much lower than either the Pentium 4-1.6A or 1.8A.
Now let's take a look at a small chart outlining the approximate prices (taken from a few large online dealers) of the various retail Pentium 4 processors:
The most natural choice would be the least expensive Northwood processor, which in this case is the 1.6A model. The 1.8A GHz retail boxed is only $15 more, while the 2.0A costs an extra $50. Granted the difference between the 2.0A and the 1.6A processors is just $50, but this difference is still substantial and could possibly be put towards a newer video card, more RAM, or a higher quality motherboard. One of the primary reasons for overclocking is to save money and to use that money elsewhere, so the higher you go on the Pentium 4 speed dial, the lower your potential savings.
In addition to the price advantage, lower clocked processors tend to give you more overclocking headroom. It is important to remember (especially those who are coming over from the AMD camp) that Intel processors are all multiplier locked, and the only method of overclocking is by increasing the front-side bus speed. The Pentium 4-1.6A uses a 16X multiplier and a 100 MHz FSB (quad pumped to 400), but raising the FSB to 133Mhz (533Mhz) would enable you to come up with a final overclocked speed of 2.13 GHz, which is a whopping 33% overclock.
Now when dealing with the Pentium 4-1.8A and 2.0A, we would then be using 18X and 20X multiplier settings, and gaining the same 33% overclocking level would be much more difficult. Not only are the default speeds already higher, but each 1 MHz increase in the FSB would raise the core speed 18 MHz and 20 MHz respectively. These 18 and 20 MHz jumps at each FSB increment really start adding up, and as you can see from the chart above, the Pentium 4-1.8A and 2.0A on the 133 MHz FSB would yield final clock speeds of approximately 2.4 GHz, and 2.66 GHz respectively.
The choice between a retail and OEM Intel processor is a battle steeped in history. The OEM (Original Equipment Manufacturer) version is sold from an open tray, does not come with a retail heatsink-fan (HSF) and commonly only includes a 15-30 day seller warranty. Compare this to a retail Pentium 4, which features a 3-year warranty, includes an Intel HSF unit and is sealed in a cardboard box, which is also covered in plastic.
Naturally, the first inclination is to go with the retail processor, since it is sealed and includes a HSF. These do tend to be more expensive than the OEM versions, but the money is well-spent when you observe the sheer number of pre-tested Pentium 4-1.6A at 2.1 GHz/motherboard combos that are presently for sale. Even though the numbers would be small, there have to be at least some Pentium 4 1.6A processors that fail the 2.1 GHz overclock and buying OEM gives you a far greater chance of receiving one of these runts of the litter.
For this reason, we highly recommend picking the retail option and getting a pristine, sealed and untested CPU. Also be wary for retailers that sell retail Pentium 4 processors that are pre-opened for whatever dubious reason, as these are as suspect as OEM tray versions and may even be customer returns. When buying retail, it never hurts to confirm that the CPU will be fully sealed upon delivery.
This is the general rule we like to follow when buying highly overclockable Intel processors, but in the current market, it is increasingly difficult to even find OEM versions of the Pentium 4-1.6A, 1.8A or 2.0A chips, so for buyers of these models, the choice between retail and OEM may well be a moot point.