I just did an upgrade of one of old (toy) servers, last week. Upgraded from an old Duron 800MHz to a Pentium D 915. I found it necessary to rework the cooling systems, to both keep it quiet and "cool".

The biggest improvement was had by drilling an array of holes above where the CPU fan draws it air, and gluing ~90mm round "duct" from the side cover (where the holes were drilled) down to the CPU. The idle temp dropped from ~50 Degrees C and 2600 rpm, down to ~44 Degrees C and 1900 rpm. On CPU burn test (Sandra) the temp tops out at ~54 Degrees C (Can't remember the fan speed though.

You can see an example of what's needed on page 6 of [this document]. Unlike what the document suggests (no I haven't read it), my CPU duct is attached to the side panel, along with a stopper screw, so when I slide the cover off, the duct isn't pried off.

I was thinking about writing the up the upgrade, but I'm not really motiviated at this point. Perhaps later after decent holiday/rest.