You set the shut down temp in the bios. Athlon 64 cpus don't have a "throttle down" feature like Intel cpus. I got my old 2.8c up to 80 celcius once before it slowed down. I would never let an amd cpu go over 60 celcius for any length of time. When cool and quiet is used, the cpu speed idles at about 1000, increasing under load automatically. But it's not the same as Intel. You can't try an amd cpu without a heatsink installed. It's still possible they will overheat and die within seconds if the shutdown feature in the board bios is disabled. With Intel, it's more difficult to destroy the cpu by running without a heatsink.
According to the manual of my amd cpu 10 seconds without a heatsink and ure screwed.