I guess your processor will operate up to 85C and maybe more. I would not expect a desktop or tower to actually run at much more than 50, or about 15-20 degrees above room temperature.

If you hit 75C, your system will shutdown. The reason for this is that it is probable that your CPU fan has failed if this temp is reached. It is an arbitrary limit, but designed to give enough time for protection. I personally set my limit at 60C, but that is for a UK environment (you can change it in the BIOS by running setup).

Hope that this helps