I'm using Artic MX-2 thermal compound. Its very good but be aware, because is a little capacitive, dont touch the capacitors near de CPU.

I don't know the normal operating temperature now. Because a have several parts of MDD G4 (3 heatsink, 3 CPU módules, 3 motherboards, 2 cases etc..). I was experimenting overclocking up to 1.7 Ghz. When I have to test the heatsinks one year ago, just swap the 3 ones in the same system and only take note of the diferences. I don't remeber the absolute values.

Now I write this from a 1.5 dual at stock voltage (1.42 overcloked). I have it for a year whithout a problem due to verclock. I don't have OsX instaled so I can't read the actual temperature.

More over, I usually put a second little fan in to the back of DVD-ROM that sends air to the heatsink, completing the original fan,cooling very well this machine, and its very easy to install.

But overcloking, core voltages, temperatures and cooling MDD may require its own thread

Sorry for my bad english.