My friend uses an AthlonXP XP2000+ with the retail cooler. The temperatures reported by the motherboard was around 62C under load (so the actual die temperature was probably even higher). Even though the temperature was extremely high, his computer could complete 24 hours of Prime95 without any problem.
Anyway, I had an old Alpha 6035 with 27cfm fan lieing around (it has the copper insert at the bottom). I replaced his heatsink with the Alpha, and experienced some wacky results.
-> Prime95 would always report a rounding error when the CPU reaches 57-58C (as reported by the motherboard).
Once I put the retail cooler back on, everything was fine again (though the temperature was back at 62C). Anyway, I'm probably going to get a Volcano 9 for his computer.