I downloaded FliteDeck to monitor the system during some tests I wanted to do. However, I couldn't get the temperatures read out... In the end, Motherboard Monitor did the trick.
During idle-time, CPU temperature is a cool 36 degrees. During DVD playback, the temp maxes out at 48 degrees -- not too cool, but certainly not too hot either. CPU is about 50% in use at that time.
When I run a DivX in full-screen mode, CPU usage shoots to 100%, with CPU temperatures between 60 and 65 degrees. With a maximum temperature rating of 70 degrees, that seems to be cutting it a bit close -- though the CPU can survive higher temperatures.
However, when I put the resolution down to 800x600 (which is not a problem at all, because we will be watching this on our 720x486 PAL television), the CPU usage drops down to around 60%, and temperature maxes out at 55 degrees.
All in all, very decent temperatures -- but this is still with the supplied PSU that blows the warm air out of the case. I think I'll try to install the fanless PSU tomorrow and do some more measurements. Then it will also become apparent just how much noise the 120S is still making.