Just sharing some tech info/warnings about cooling the Peg2 G4:

Heat-pipes use gravity to return condensed fluid back to the dissipator. So they function only when the heatsource is lower and hotter than the other side of the pipe.

I don't know about all Pegasos II G4 cards, but mine has a MPC7447 1000LN fitted to it. This G4 is an industrial version which breaks down at a junction temperature of 105 degrees Celsius. To be on the safe side you should keep its maximum operating temperature well below 35,2 Celsius lower, hence = 69,8 Celsius.

Never use any of the northbridge (or even Alpha) coolers without a fan! As the CPU can dissipate 22Watts at maximum workload, it will fail within minutes if it's not properly cooled.

For passive cooling you need a heatsink with a thermal resistance (much) less than 1.2 K/W. These are hard to come by. Besides being heavy and difficult to mount, they will only work adequately when mounted vertically (requirering a spacious horizontal desktop case). So second best is to have a large (12cm) case fan for cooling. Large fans run quieter.

If you live in a not too hot climate, you *might* get away using only a passive videocard heatsink and active case cooling.

An example (be carefull as this will void your warranty and you might damage your computer beyond repair):
I experimented myself with the front half of a Zalman ZM80A mounted as heatsink to the G4 card. I extended the base a little with copper sheet so that the heatsink floated above the capacitors of the G4 card. I removed the original fan from the G4 card, which I found too noisy. Like many of you.

I used the rear half of the Zalman as a heatsink for my Radeon 8500LE. Which runs cooler than I'd expected.

I disconnected the (also noisy) case fan from my Coolermaster ATC 620 BX desktop case. I installed an ultra low noise 300W ATX PSU which has an internal horizontal, top mounted 12 cm fan. The PSU has quite open honeyrate backplate, allowing good airflow.

With a closed case the CPU temparature was 54-57 degrees Celsius (running a DVD, 60% CPU load). This is almost acceptable as it means I can use my peg2 upto 33 Celsius room temperature. The PSU fan seems to generate sufficient airflow throughout the case. It even makes less noise then the whispering Maxtor 160GB. Overall, the whole system is as quiet as a laptop.

Just to see how much of a difference the airflow makes, I left it to run with an open case. Not a good idea. The temperature reached 69 Celsius in less than 30 minutes and was still rising... So I switched it off before I'd to buy a new G4. ;-(

Oh, the ambient temperature was 20.1 degrees Celsius. No fancy software was used for measuring temperature. Just a digital thermometer and sticky tape.
For the record: No animals or electronic parts were harmed by this experiment. But perhaps luck was on my side