core temp and TAT monitor temps based on a sensor built into the cpu itself. Other programs like speedfan and bios monitor temps from a diode on the board and then uses an algorithm to calculate what your cpu "should" be at. If you use TAT, the temps would be similar to core temp. Now for core 2 on stock cooling, I'd say temps anywhere between 35-45 is rather normal depending upon case vent. the 85C intel put out a threshold temp is based on TAT temps and coretemp's reporting is the closest i've seen to TAT.
I though Core Center checked on the inside diode. But if this is the case. What temp am I supposed to follow when I OC to keep it below 60C?