If you trip the thermal limit, the machine will just likely shut off instantly before the part is damaged. It's also not unusual for the driver to crash if the hardware is extremely hot.
When my GTX 260's fan died, I was pushing nearly 110c with even the most minor activity. The driver would crash continually and if it kept climbing, poof, computer would just turn off. The metal backing was actually so hot that I probably could of burned myself on it. Check your temperatures!