@Andreas_Wolf

Sure, other G4's exist. But the Pegasos-2 G4 only shipped (as far as I know) with the 7447 1GHz, which is a rather hot chip.

I would definately recommend passive cooling. Unless you have a very large and properly secured heatsink. However, that could run into other problems such as causing too much stress on the vertical CPU card.

Best to get a heatsink/fan to replace the stock one that is perhaps a little bit bigger, but still with fan. Copper is a good metal for the base of the heatsink.