excerpted from an article by Joe at O/C.com... best site around without permission but to the point... plenty more good info to find too...
"AMD's specs contained in "AMD Thermal, Mechanical, and Chassis Cooling Design Guide" (p. 13, Publication # 23794 Rev: B Issue Date: October 2000) gives the following guidelines:
* CPU up to 48 watts: C/W of .55
* CPU up to 55 watts: C/W of .41
* CPU up to 70 watts: C/W of .32
C/W means that for every watt the CPU radiates, the heatsink is supposed to cool the core by C/W x watts plus ambient temp. AMD specs system temp at 45 C, which is on the warm side. What all this means when you run the numbers is the following:
* CPU up to 48 watts: 26.4* + 45 = 71.4 C
* CPU up to 55 watts: 22.6 + 45 = 67.6 C
* CPU up to 70 watts: 22.4 + 45 = 67.4 C
*(48 x .55)
Now that's AMD's Guideline; if you're overclocking, my experience has been that temps in the mid 40s and up will start to impact stability. Temps per AMD are OK at normal speeds and voltages, although anyone used to Intel's temps has a hard time when they see numbers like this - I know I do!
So if you just adhere to AMD's spec, anything over 67 C is not recommended. Hopefully you do better than 45 C case temp, so you should knock at least 10 and better yet 15 C off the 45; this gives you something like 50 C, assuming a 30 C case temp. That's about the upper end of "acceptable" from our perspective.
It's no surprise that we are seeing a lot of heatsinks coming with Delta 38s - how else can they deliver cooling temps that we consider acceptable with older heatsink designs? This new crop of "screamers" is pushing C/Ws into the 0.25 range; this means, with a case temp of 30 C, CPU temps of 47.5 C at 70 watts is achievable.
For my taste, temps between 35 C and 45 C are achievable and recommended. "