There was like a 12-13 degrees C difference at full loads when I tried both cpus with the same PEP66T (Don't remember perfectly, but I think it was like 52C Vs. 39C). To be honest, mine were overclocked chips; I just happened to run them at these mentioned speeds for testing (900Mhz TBird at 1Ghz, and 750Mhz Duron at 950Mhz/1Ghz). Both of the chips were run with 1.85-1.9V.