Kositch,
There are several variables involved, so allow me to put the big picture into perspective for you.
Core 2 Duo's have a single CPU temperature sensor (Tcase), and two Core temperature sensors (Tjunction). For the E8400 C0, Intel's Thermal Specification is 72c, which is shown in their Processor Spec Finder -
http://processorfinder.intel.com/details.aspx?sSpec=SLAPL
72c is maximum CPU temperature (Tcase Max), NOT Core temperature, which is a common misconception among many users. Also, there's a 5c Gradient between CPU temperature (lower) and Core temperature (higher), which is shown in the following Intel document -
http://arxiv.org/ftp/arxiv/papers/0709/0709.1861.pdf
Although maximum Core temperature (Tjunction Max) is 100c, which is for Throttle and Shutdown protection, the corresponding CPU temperature would be 95c; far too hot for sane operation. As such, CPU temperatures above 72c and Core temperatures above 77c should be considered as an "overtemp" condition.
SpeedFan shows CPU temperature AND Core temperature, while Real Temp shows Core temperatures ONLY. Both programs allow calibrations. Intel has stated that the Digital Thermal Sensors (DTS) used for monitoring Core temperatures are accurate at very high temperatures, become less accurate as temperatures decrease, may "stick" below 50c on 45 nanometer Core 2 processors, and may be unreliable at idle temperatures.
On the other hand, the Analog Thermal Diode used for monitoring CPU temperature is linear from idle thru high temperatures, which is why, as per agreement with Intel, motherboard manufacturers do not include Core temperature in their monitoring utilities found on the installation disk, such as Asus Probe.
Never assume that default temperatures are accurate. The accuracy of CPU temperature is determined by BIOS calibrations, but can be closely calibrated in SpeedFan. The accuracy of Core temperatures are determined by Intel factory calibrations, but can be closely calibrated in SpeedFan and Real Temp.
Prime95 Blend, or OCCT (Linpack), or CPU Burn Test (LinX) are cyclic workloads, which produce fluctuating temperatures. While these are useful for stability testing, they're inappropriate for thermal testing. Prime95 Small FFT's is the standard for thermal testing, because it's a steady-state 100% workload. Since thermal saturation is reached within 7 to 8 minutes, a 10 minute test is adequate. Keep in mind that even the most processor intensive games or applications will rarely exceed 70% to 85% sustained workload.
From the
Core i7 and Core 2 Temperature Guide:
http://www.tomshardware.com/forum/221745-29-sticky-core-core-temperature-guide
Scale 2: Duo
E8x00:
Tcase Max
74c, Stepping
E0, TDP 65W, Idle
8W
E7x00:
Tcase Max
74c, Stepping
R0, TDP 65W, Idle
8W
E7x00:
Tcase Max
74c, Stepping
M0, TDP 65W, Idle
8W
E5x00:
Tcase Max
74c, Stepping
R0, TDP 65W, Idle
8W
E5200:
Tcase Max
74c, Stepping
M0, TDP 65W, Idle
8W
E4700:
Tcase Max
73c, Stepping
G0, TDP 65W, Idle
8W
E4x00:
Tcase Max
73c, Stepping
M0, TDP 65W, Idle
8W
E2xx0:
Tcase Max
73c, Stepping
M0, TDP 65W, Idle
8W
E8600:
Tcase Max
72c, Stepping
E0, TDP 65W, Idle
8W
E8xx0:
Tcase Max
72c, Stepping
C0, TDP 65W, Idle
8W
<--E8400 C0
E6x50:
Tcase Max
72c, Stepping
G0, TDP 65W, Idle
8W
E6540:
Tcase Max
72c, Stepping
G0, TDP 65W, Idle
8W
-Tcase/Tjunction-
--70--/--75--75-- Hot
--65--/--70--70-- Warm
--60--/--65--65--Safe <--
--25--/--30--30-- Cool
CPU temperature = Tcase
Core temperature = Tjunction
Also, Vcore Max for your E8400 C0 is 1.3625
Hope this helps,
Comp