GPUs are more tempermental when it comes to increasing voltages than CPUs. also, temperatures are probably not going to be the reason behind a dead graphics processing unit but, rather the voltage (or the amount of current) being pushed through the GPU. however, for Kepler GPUs (GTX 6xx series cards), the GPU will start throttling or decrease it's clock speed by 13MHz when it hits 70C.
that being said, the limits of your overclock will also depend on the GPU itself, some GTX 680s will push further while others won't budge from the stock values.