I bought an Athlon X2 5000 Black Edition (unlocked multiplier) this week at MWave for $70 to replace an X2 5200 that I'm giving to one of the kids to upgrade their X2 4000. Both the 5200 and now the 5000 run warmer than I expected for 65 nm CPUs - in the high 40s-mid 50s ºC range. I tried the stock AMD cooler as well as an ArcticCooling Alpine 7 HSF, no difference. (I expected these to run in the mid 30s or thereabouts.) Both HSFs make firm contact with the CPU, the thermal paste isn't under- or over-applied and the smart cooling is disabled in BIOS to allow the fan to run full-speed.

I'm still using what's left of a tube of ArcticSilver 5 that I bought three years ago. Can that have any bearing on this little puzzle? Other question: has anyone tried mounting the heatsink fan backwards, drawing air from the CPU upward rather than blowing downward onto it?