On thoroughbreds: Personally, I would wait until we get thoroughbred "B" cores, which is what the 2400 and 2600 are based off of, at lowend speeds. That way you get the chip very cheap, highly OCable, and don't have to worry about heat asmuch as the thoroughbred "A" cores out there now.
Simmike2: First of all, FC-PGA2 is a way of connecting a CPU to a motherboard. It has nothing to do with whether there is a heatspreader on the chip or not. And also, contrary to its name, a heatspreader does NOT help dissipate heat. It might spread heat evenly over the heatsink, but unless you are making your heatsink out of steel or something worse than aluminum, that is not likely to be a huge factor. The primary purpose of a heatspreader is to keep the core from being chipped on application of a heatsink. As a result, Intel is free to make heatsinks such as their retail boxed HSF, which clamps so tightly onto the processor that it actually bends the motherboard about a quarter inch, deforming an area around the processor. A heatspreader will never improve the thermal contact between HSF and chip, as it is merely another barrier between the heat source and the heat dissipator. The only reasons that Northwoods are so easily cooled are:
1) They are .13micron and built so that they produce very little heat compared to .18micron based processors
2) The die area of the northwood is very large in and of itself, and the heat comes off the processor evenly over a large area, instead of over a tiny area as with the relatively tiny core of the Thoroughbred, through which 70 watts of heat is put out. This is partly because economically, AMD and Intel are more successful if they put out more processers per wafer. AMD shrunk their processors to do this, Intel switched their wafer size from industry-standard 200mm to 300mm
3) The retail HSF that intel put out with the Northwood was very well built, a very effective design, and is probably the best retail HSF boxed with any processor, as compared to the aftermarket coolers. Because of this, there has been very little activity in the P4 HSF market compared to the Athlon HSF market, with something like 5-10% of the cooler models as the athlon.
On 24C temp readings: 24C is, FYI, 75 degrees fahrenheit, which is about room temperature in most locales. Accurately read, even the best non-peltier, non-phasechange watercooling setups increase the temperature atleast 2C-5C degrees above ambient at the waterblock. If the motherboard is reading the under-chip diode(built into the motherboard), then it can be off by as much as 10C-15C, if it is reading the on-chip diode(which your motherboard isn't) it is considerred the most accurate reading. I don't think this is your problem though, I think somehow your motherboard is reading case temperature or something. Even on an under-chip diode, it can't be at room temperature if the processor is on.