Everest Ultimate or If you have a Gigabyte MOBO the Easy tune 6 will also shoe the CPU temp correctly (though the individual core results show as 0C - it happens on all of the CPUs when you unlock the extra core (including the X2 550 also ) so is normal !
I've got my X3 running at 3.2 GHz. and the 4th core and it has had no problems (I've been running it with the core unlocked since Mid December @3.0GHz. but the temps were a bit high on the stock fan - but I just got a new Heatsink\fan so have it running at 3.2 now (will be testing a higher OC after awhile -letting the new Thermal paste set a bit before pushing it ) - Though the temps are already much better (stock HS was 45C idle 70C Prime95 @ 3.0Ghz - Cooler Master TX3 (got it for $4.99 from newegg - $14.99 sale price and $10 Off next purchase Code) Temps are 35 C idle and 52 Prime95 @ 3.2 GHz. !!) - Here is a Screenie of Everest and Easy Tune 6 showing the CPU temp correctly --