I've been doing some tests during the last week and currently I've two 2Pacs running at 225 MHz, cooled by a rpm monitored 12cm fan, providing about ~25 GH/s each one.



The fan has a resistor added to lower the voltage from 12v to 7v, so it's very less noisy. Even the reduced speed and the 2Pacs being overclocked, the heatsinks are never not too hot.
I wrote a script to monitor the fan every second, so if its speed drops below 2000rpm, the cgminer process will be automatically killed.

And finally, I've edited the miner.php included with cgminer to show the speed at the title bar, check if the script that checks the fan speed is running, and the current fan speed.
After a few days doing some testings, now it has been working for more than 24h.


The little computer is an old Astaro Security Gateway, running a Debian Linux, that luckily has 3-pin fan connectors inside, so I can use them to add that external fan and monitor its speed  
The USB hub is powered by a 5v 3A power supply.

Power consumption of the computer is about 25W plus the miners.

If anyone wants more info about the mine.php modifications, or the scripts to check the fan speed, just ask