The maximum safe temperature for a processor varies widely by model, as does the temperature the system will shut down at. After a bit of searching, the maximum temperature before you start doing immediate damage is 105C, but you never want to get it that high. From intel's datasheet, the temperature the CPU shuts down at varies slightly with each system. Various other places suggest that it's around 95-100C, which is still higher than you really want to be running the system at. Running at a constantly high temperature isn't good for the CPUs lifetime. You should be fine if your temperature peaks around 80C, although I wouldn't try to push it much beyond that.
For comparison, my laptop has a core 2 duo @ 2Ghz, it's safe up to 100C but automatically shuts down at 85C. Idle temperature is usually around 60C and its stable between 70-80C after several hours on full load. Mobile parts have a tendency to run warmer than desktop parts though, so I'm not sure how useful a comparison that is.