66c is not going to hurt anything, but it is still warm. That's about what mine is running at right now, maybe 1-2c higher at peak during stress. While you might not kill anything, you may shorten it's lifespan
I'm not worried about mine, that is why I let it run that warm. This 6750 was an interim O/C project/learning experience until the cheap Penryn quads hit the channel.
Another thing about overclocking. Don't just get it stable and be satisfied. Keep notching the voltage down and stressing it until instability creeps back in. Then, just set it to the lowest stable voltage, and stress it for 24 hous. If it passes, that's your voltage.