Do you know if your SpeedFan CPU and Core1/2 temps are appropriate and proportional?
Speedfan only gives temp readings for the CPU as a whole, not for the two cores separately.
I don't think any of those utilities are completely reliable. I use speedfan because:
1) its readings are the same both under XP and Vista
2) TAT readings vary wildly from XP to Vista (20C difference!!!), not to mention that under XP they seem too high (~45C) and under Vista they are very low (~19C-23C)
3) Coretemp only works with XP and the readings seem too low to be true (~19C-23C)
4) Speedfan readings are the same as the ones in Asus utilities
5) Bios readings are just 5C above Speedfan readings.
Taking all this into account I'd say that my "real" iddle temps are somewhere between the Bios readings and Speedfan.
If I remember correctly that Guide to C2D Temps says that Speedfan gives lower temp readings than Coretemp; in my case its precisely the opposite.