Does anyone know how big a fan this thing can handle before it becomes a burden on the power supply? I was thinking of popping the circular labeled piece of plastic off the of top and mounting a 60mm fan there....
Have a look at http://bzed.de/posts/2010/09/refactoring_the_guruplug_server_plus_-_part_two/
If you add a 60mm to 80mm fan adapter you might even use a silent 80mm fan.
Make sure you remove the heat spreading piece of alloy and maybe add some holes to let the air in/out. Then a slow fan should be enough to keep everything cool.