I have also heard that some of the Northwoods are at a higher power consumption than others.. not sure as to the truth to this.
I was pushing 56-58C under load using the stock heat sink with the thermal pad. I removed the HS to see if it was making contact, and there were marks in the thermal pad coresponding to the P4 heat spreader... so it was making good contact all the way around. I then purchased some AS3, applied a thin layer, and reseated the HS. My temps dropped to around 55Max. About a week later, someone suggested I remove the HS, clean off the AS3 on the heatsink, smooth, rub in the AS left on the P4, and reseat the HS... effectively cutting the layer of AS in half (even though I initially just applied barely enough to cover the writing.).
Anyway, I saw another temp drop of 1-2 degrees. I barely have any at all on there now.
My HS gets a little warm, but not too much. I stopped worrying about it.
Mine also will not go past 2.1... unless I set the RAM to 3X, so I think my DRG's or whatever they call them max out at 450Mhz or so. I don't have the soldering skills to replace them with higher rated ones.
I just think some are 2+ Ghz chips thrown into the 1.6/1.8 basket to meet demand, while others are true 1.8's