I fount these useful links, that state that the mac mini only starts to really speed the fan around 75°C :
http://forums.macrumors.com/showthread.php?t=943010
http://discussions.apple.com/thread.jspa?threadID=2287115&tstart=0
I checked (using iStat nano) after playing a few minutes to a game, and the fan ran up to 2000 rpm from base 1800. I also remember that after playing a long time I finally heard the fan (not audible before).
I checked (using temperature monitor) the temp, it was up to 52°C for processor (from base 40), and up to 50°C (from base 38) for northbridge (graphic chipset) after that short playing, and after quitting the game the fan was down quickly to 1800 rpm. The temp took about 8 mn to be down to 45°C for processor and 43°C for northbridge (power supply remainded at 60°C), so it should be ok after 15 mn.
I installed smcFanControl instead of Fan Control, as the later installs a daemon (and I don't want). However after agreeing (it stated that smcFanControl hasn't been tested on this machine, however that it should run fine), it did nothing... A process was here (with daemon as a parent !) however no window was displayed (no interface). I shut down the mac mini, waited a little, and restarted it, hopefully no smcFanControl process anymore.
I wonder if smcFanControl has modified some system files and could have hurt the system ? How to check all is back as before ?
Launching a game later I had a feeling it was slightly less smooth, however I may be wrong (that game was very sluggish a day ago after a heavy use - that could have lead to memory fragmentation -, and was ok after a restart). So it should be just the fear that smcFanControl had broken something.