Duron case temperature can resist upto 90C.
Take it lower for safety, but most overclocked Duron/TBird temperature range mostly between 50-55 which is still considered very normal.
Even without overclocking Duron runs at about 44-45 C with bad heatsink, with TBird 800-900 can run at 50-55 C.
So, I believe as long you are still below 60 C, you are still very safe.