My mistake... the Palomino XPs do have a 90*C maximum die temp. I must've been thinking of something else...
Regardless, I keep hearing that the Athlons tend to get somewhat "flaky" above about 60*C... although I have no firsthand experience with this, and very little hard evidence of it, it comes up fairly often. I do have one case I can point to that I can think of off the top of my head, which is the review of the Aerocool Deep Impact HSF that Viper Lair recently did. The Deep Impact is a heatpipe-based heatsink, and they as part of their review, they ran it without a fan, basically to see why the temperature difference between the stock 14cfm fan and a 40cfm Delta was only 2-3*C. With passive cooling, the CPU they used in the test, a 1900+ @ 1.75v & 1.67GHz, stabilized @ 61*C. At that temperature, it idled fine but would not boot into windows. Relaxing the OC apparently did not help.
The review is here:http://www.viperlair.com/revie...cool...ool/deepimpact/
However, I did notice they used an ABIT AT7 Max for the test, and I don't know how the AT7 reports CPU temps in the BIOS- that is, I don't know if 61*C is the in-socket temperature, or the CPU temperature as reported by the CPU's internal diode.
If it's the socket temperature, then the actual CPU temperature was probably around 76*C (In my experience, the difference between actual and in-socket temperatures is around 15*C).
As for why it makes me uncomfortable, it's basically a reliability issue. A smaller HSF generally has less cooling capacity than a larger HSF does. Less cooling capacity means less reserve capacity, which means that it's less well equipped to deal with varying environmental conditions.
If you think dust only raises temperatures by 10*C, if that, then you haven't seen some of the computers that I have. I have one friend who insisted on using the retail HSF, even after I told him that I didn't think it was a good idea. He's got a 1900+ Palomino, so it's got that dynatron skived aluminum HSF. I took that system apart about a year after he bought it, and that was the first time it had been opened since it was bought. The top of that heatsink was literally covered with dust. the vast majority of the air was going through the channel in the center of the heatsink that was left open for the spring clip. Because the fins on these newer heatsinks are spaced so closely, the dust had literally just built up on top of the fins, like it would on an air filter for a case fan or something... very little of it actually went into the fins themselves. I've taken apart older P2 systems where literally all of the space between the fins on the heatsink was packed with dust. So dust can have a major impact on the performance of a given HSF. And, since in many cases, the AMD retail HSF performance is marginal at best, making it deal with that kind of dust is just asking for trouble.
Beyond dust, many people still don't have air conditioning. If it's 90*F in a room, the intake air temperature for the CPU fan in a typical midtower (no intake, no exhaust except PSU fans) is going to be in the neighborhood of 100-120*F. To keep the CPU temperature below the absolute maximum allowable temperature IMO (150*F) would require no more than a 30*F delta t between the CPU temp. and the intake air temp. If you assume a worst-case scenario for an AMD-based system, the CPU would be either a 1.4 Tbird, 2100 Palomino, or 2200 TBred A, all of which put out a bit over 70w at most, as I recall.
Thermal resistance = delta t/input power; to handle one of those CPUs in those kind of conditions would require a thermal resistance of ~0.22*C/w to ~0.38*C/w, depending on intake temperature and assuming a 70w heat load. According to Dynatron's specs for what I THINK is the retail HSF that was included with the Palominos (http://www.dynatron-corp.com/p...cooler_model.asp?id=1)
the thermal resistance is 0.49*C/w. Given a 100*F/38*C intake air temperature, a thermal resistance of exactly 0.49*C/w, and a heat load of exactly 70w, the CPU temperature would be 72.3*C, which is within it's design limitations, but which is hot enough to potentially cause stability problems.
It's simply a matter of preferring more headroom for varying conditions (dust, fan failure, use in extremely hot environments, etc.) than the stock AMD coolers have provided, with the exception of this Ajigo unit they're using now.