I had my duron 600 @1000 Mhz but temperatures exceeded 55 degrees C. With the case open temps didn't exceed 49. I took the thermal tape off my heatsink and replaced it with thermal compound. I also installed a second case fan but I still couldn't get the CPU to operate below 50 during hard use and with the case closed. At 950 Mhz It still crossed 50.
I did have one crash when the temperatures were running high and I had Need for Speed 3 just quit on me and drop me back to the desktop. Since I can run the system all day with the case open I have to attribute those problems to excessive heat.
I think I am going to settle for 900 Mhz where temps are between 40 and 47 degrees. Besides, with only a Geforce256 SDR there is not a whole lot of performance difference between 600 Mhz and 1000 Mhz, 4500 3DMarks vs 4950, nevermind between 900 and 1000 Mhz.
By the way my MoBo is a KT7 and I love it.