I found the probable cause of the not so ... / 2010-02-09

I found the probable cause of the not so great power saving: when I installed the first new disk I also updated the bios. And the message I get when trying to load the powernow-k8 cpu driver is:

powernow-k8: Found 1 AMD Athlon(tm) Dual Core Processor 4850e processors (2 cpu cores) (version 2.20.00)
powernow-k8: MP systems not supported by PSB BIOS structure
powernow-k8: MP systems not supported by PSB BIOS structure

So the cpu keeps running at maximum speed without throttling. Searching for the error message finds Ubuntu Bug #33116: powernow-k8 refuses to load and Ubuntu Bug #398109: powernow-k8: Your BIOS does not provide ACPI _PSS objects in a way that Linux understands suggests that I need to check the bios settings to enable "Cool'n'Quiet", enable ACPI APIC and disable MCP61 ACPI HPET Table. That's planned for the next hardware changes.