Hi,
Based on 8W of power @ 40Mhz, the maximum temperature, measured on the case would be 85C. There is a Junction to case thermal resistance of 3 C/W so in use the case heats up by 24C , so you simply subtract this self heating from the maximum device temperature to get the safe temperature on the case.
The hotter a part gets, the slower it operates. Depending on the design, this may causes stability issues. Cooler is better.
@Gulliver
Look at some of the new pin shape CPU coolers, they are amognst the most efficient on the market. In a towered A1200, you should have ample room for a taller heatsink. If you can fit one of thses, you may be able to reduce the number of cooling fans.
Worht looking at this software,
http://www.heatsinkdesigner.com/@Delshay.
The Maxim 1617 will read the thermal diode and provide a temperature reading via the I2C bus.
@Fanscale
I have run the MPC8458 @ 1 GHz at a junction temperature of 105C, for 24 hours with no issues. Intel/AMD cpus might expire but the Freescale/Motorola ones are hardy.
Ian
