Rule of thumb: if it starts crashing, it's too hot.
Honestly, the odds of actual thermal damage occurring are slim to none. While heat does decrease component life, we're generally talking about over the course of a number of years, by which time the computer will likely have long since been replaced anyway.
That said, my not-quite-comprehensive experience says:
CPUs start getting pretty crashy around 55c (discounting thermal protection).
GPUs get flakey above 85c.
Hard drives above 50c concern me.
Case temps above 40c can lead to tetchy motherboard components.
Chipsets always run much hotter than you think they should.
But, as always, refer back to the rule of thumb.
Honestly, the odds of actual thermal damage occurring are slim to none. While heat does decrease component life, we're generally talking about over the course of a number of years, by which time the computer will likely have long since been replaced anyway.
That said, my not-quite-comprehensive experience says:
CPUs start getting pretty crashy around 55c (discounting thermal protection).
GPUs get flakey above 85c.
Hard drives above 50c concern me.
Case temps above 40c can lead to tetchy motherboard components.
Chipsets always run much hotter than you think they should.
But, as always, refer back to the rule of thumb.