Your processor's Maximum Junction Temperature (Tjmax) will be somewhere in the vicinity of 100c. This is the point where the processor throttles its own performance to lower temperatures. We'll thus call this our "way too hot" threshold.
Your processor's Control Temperature (Tcontrol) will be somewhere in the vicinity of 80c. At this temperature, your processor's cooling fan is supposed to be running at full speed (100% duty cycle). The processor's temperature is allowed to go above this level, but only for fairly short durations of time. We'll thus call this our "too hot" threshold.
For desktop processors, you can obtain your processor's actual Tjmax and Tcontrol values by reading the IA32 Temperature Target Model-Specific Register (MSR) (address 0x01A2). Bits 16-23 of this register specify the processor's Tjmax temperature and bits 8-15 specify the processor's Tcontrol temperature offset. Subtract this Tcontrol temperature offset from the Tjmax value to get the Tcontrol temperature.
You can read MSRs using tools like RWEverything. In my processor, the value in this register is 0x00641400. This means that my Tjmax temperature is 0x64 (=100c) and my Tcontrol temperature offset is 0x14 (20c). This means my Tcontrol temperature is 80c.
As for the lowest temperatures you might see, most thermal solutions will allow you to see temperatures in the 30c-40c range when measuring at Windows idle.
Note that, if monitoring temperatures from within BIOS Setup, expect the idle temperature to be much higher, perhaps as high at ~70c. This is because all power management features of the processor are disabled while in BIOS Setup.
Hope this helps,