Nice post Matt555, i'd like to add if i may!

This post is there to answer the common question: "My CPU is running at xx degrees, is it too hot?". I've provided the maximum temperatures for some of the most popular CPUs.

Keep in mind that the onboard measurement diodes are often inaccurate and may report temperatures that are too lowor too high. This is especially the case with motherboards that use a thermal sensor below the CPU to "guess" the CPU temperature. The temperature values displayed by the BIOS have usually a correction value added, to compensate for this problem, but in some cases this correction value may be too low, or the sensor might not be in good contact with the CPU.

However, this doesn't mean that you should start to panic when your Athlon XP CPU reaches 60 degrees celsius, for example. Most people tend to
underestimate normal CPU operating temperatures - maybe due to overclockers bragging with their super-low CPU temperatures.

Also, note that these values are for CPUs that are at stock speeds and not overclocked. Overclocked CPUs may run unstable even if their temperature is way below the maximal specified temperature.

What happens if the maximum operating temperatures are exceeded?
If your cooler is insufficient and the temperature exceeds the maximum operating temperature, then this does not mean that the CPU is automatically damaged. With AMD CPUs, you will usually encounter crashes if the CPU is overheated; but these go away as soon as the CPU is cooler again. In the long term, running the CPU at a temperature that is too high may reduce the CPU life, since an overheated CPU is more prone to electromigration - even if it runs stable.With P4 CPUs, the CPU will turn its speed down automatically when it overheats. No damage to the CPU is possible, but the system will get slower while it's hot.

If you attempt to operate a CPU without heatsink at all, recent AMD CPUs will usually be permanently damaged within seconds, unless special protection circuitry is available on the motherboard. P4 CPUs will run excessively slow without cooler.

The purpose of this post is to give you a quick overview of typical maximum operating temperatures for common CPUs.
Higher is better here!

All Slot A CPUs (Athlon classic, Athlon Thunderbird) 70°C
Athlon Socket A up to 1 GHz, Duron up to 1.3GHz 90°C
Athlon "Thunderbird" Socket A 1.1GHz or more 95°C
Athlon MP 1.33GHz or more 95°C
Athlon XP up to 2100+ 90°C
Athlon XP 2200+ and faster 85°C
Duron "Applebred" 1.4G and faster 85°C
AMD Opteron 70°C
Athlon 64, 64FX, Sempron
Most models 70°C; 65°C for some Socket 939 Athlon 64 models
Athlon 64 X2 (dual core) 65°C

The temperatures specified for AMD CPUs max case surface temperatures.

Pentium III Slot 1 ('Coppermine') 500-866MHz 80-85°C
Pentium III Socket 370 and Slot 1, 933MHz 75°C
Pentium III Slot 1 933MHz 60°C
Pentium III Slot 1 1GHz 70°C
Pentium III Slot 1 1.13GHz (first version) 62°C

Pentium III/Celeron max temperatures are the maximum temperatures reported by the thermal junction inside the CPU.

Intel Celeron / Celeron
Celeron 266-433MHz 85°C
Celeron 466-533MHz (0.25µ) 70°C
Celeron 533-600MHz ('Coppermine) 90°C
Celeron 633 and 667MHz 82°C
Celeron 700-850 MHz 80°C
Celeron 900MHz-1.4GHz 69-70°C
Celeron 1.7GHz and faster 67-77°C

Intel Pentium 4, Pentium 4 Extreme Edition, Pentium M
Max. temperature depends much on model and clockspeed, but no clear pattern is visible. Consult Intel's tech specs for information on your particular model.
(Lowest: P4 EE 3.2GHz with 64°C, highest: P4 1.8GHz with 78°C) 64°C - 78°C
Pentium M 100°C

Intel Pentium D (dual core)
Pentium D 820 63°C
Pentium D 830 and 840 69.8°C

Note: Pentium 4 and Pentium D temperature specifications indicate the maximum cover temperature, which is usually lower than the temperature
reported by the internal thermal diode. Therefore, your system may be running fine even if the reported CPU temperature in the BIOS is higher than the temperature specified here. This does not mean that you're on the safe side, though.