The maximum recommended temperature for your processor is 67.9 degrees Celsius as showing at:
However, please keep in mind that you do not need to worry about the temperature per core, you need to check only the CPU temperature, not the cores temperature.
As long as the CPU temperature is below 67.9 degrees Celsius, the processor will be running under thermal specifications
Al, A good free hardware monitoring and information program is HWiNFO, which is available for download here:
When using this program, you'll see the CPU and the individual core temperatures. The core temperatures on Intel i7-900 series CPUs can be about 10C higher than the single CPU temperature. That is normal and nothing to worry about. You'll find the core temps change quickly depending upon their level of activity, which is also normal.
Depending upon your mother board and the sensor chip it uses, you'll see many other things listed in the HWiNFO Sensor window, such as your chipset temps, voltages, power usage, fan speeds, and video card information if you use one. Some of the names or titles of what is displayed are not exactly correct and are just generic terms, you can change those if you like. You'll need to experiment with it for a while to understand it and use all it's features. If you run two or more hardware monitoring programs at the same time, it is common for one of them to read and display incorrect values, particularly for temperatures, so watch out for that.
While these programs are helpful they can also make you nervous, as you may find some surprising temperatures of components besides the CPU.