I would guess that SpeedFan shows the most accurate cpu-temp of those programs. Btw, I get the same reading in Sandra when i use "Environment Monitor", it draws a graph and it seem to match SpeedFan well. I don't know where you got your numbers from ? HM don't work at all for me, it just fluctuates, I believe it is actually reading the aux-temp. Why ? I don't know. The programmer should know ...
Seems a little hard to have to clock the board till it freezes, just to find out ehmm when it freezes, that's why we have temp-sensors to avoid uneccesary crashes we don't provoce by ourselves when we overclock to much. And the case gets dusty inside after some time which drive the temps up, it is nice to be warned when to clean the computer inside before something crashes from heat. It could be important work unsaved.
btw, my box is completely stable at 2475MHz at the moment, I won't clock any higher until I get my new memory. I never got the old memory to work in dual channel, reflashed several times using all the most powerful commands to rewrite bios-bootsector, clear dmi-pool etc, cleared cmos etc no-go then it stopped working with two sticks in single-channel, I tried the sticks one after one and it looks like one of them is dead. Strange though, the remaining one is only able to boot in slot 1 - where it functions perfectly once up and running - but shouldn't it be possible to boot from one of the other slots as well ?