i have a 5200x2 and a 6000x2 and found the stock hsf on both to be shockingly bad at ntune clock levels they could reach the late 60's amd told this was fine, i use an akasa evo 120 on each of them now and even when its warm they wont go ove 30c (with the fan set to lowest) i have also replaced the hsf on my 8800 graphics cards because the stock coolers again are hopeless ok they keep within 'manufatures guidelines' but personally i think if a add on cooler halves the temps there is something very wrong.
Opteron 180 (X2 4800+) here. And on the stock heat pipe cooler, 30C idle and 46/48C under stress (using test program SP004-ORTHOS). Even overclocking to 2.8ghz, I only get around a +5/6 degree increase. And ORTHOS will stress your CPU a damn site more than you are ever likely to see it stressed in "real life".
My experience is that it's usually more to do with how good/bad the HSF has been seated on the CPU (especially if people apply their own thermal paste ... badly). Though I have come across the odd case of the IHS not make decent contact with the chip and in that case, you'll never get optimum cooling.
Though obviously ambient room temp and case ventilation also contribute to overall temps.
The request for information was due to me upgrading to the above chip and getting rather higher temperatures than I would have liked. At around 53c, although within the manufacturers tolerance, it was uncomfortably high.
I've now fitted an Arctic Cooler Freezer 64 Pro and the temperatures dropped to around 41c during normal use. I expect that once the Arctic Silver compound "beds in", it will drop a little further, but in any case, a lot better than it was.