What is the maximum temperature my CPUs can reach before causing damage?
According to Intel your processor Tcase(*) should be fine at temps up to 71.4°C
(*) Tcase temperature is measured at the IHS or that metal casing protecting the core on your processor, which is what your BIOS will report. Core or Tjunction temperatures will show a higher temp.
My iMac Core2Duo T7700 2.4GHz is currently running both cores at 100% (H.264 encoding) and they both read 72°C. Fans are quiet (as usual on this system) and everything's smooth.
This system is three years old and more often than not spends one or two days encoding lots of stuff.