Hello!
Now that my experimental box/file server has almost been finished, I think it's time to show you some pictures and to give out some information.
First, the hardware the box is consisting of:
CPU: AMD Geode NX 1750@14W
CPU cooler: TT Silent Boost K7 (fan removed)
Motherboard: PCChips M811LU (VIA KT266A chipset)
Memory: 2x 512MB Infineon PC2700-2533
HD: WD Scorpio 40GB 2.5" (decoupled)
CD: LiteOn 16X/48X DVD-ROM drive (loud, but rarely used)
Graphics card: ATI Radeon 9100 128MB @ 250/200 (fan removed)
Power supply: Morex 120W DC-DC fanless power kit*
Fans: None so far...
Some pictures:
Front view of the case (a generic aluminium one), you can see the DVD drive, the fan speed controller/temperature sensor and the decoupled WD Scorpio hard drive (however, its bay is normally closed).
The inside of the case. There's not much inside it, anymore

All case fans (a 40mm on the back, an 80mm in front of the HD cage and an 80mm in the left side panel) have been removed. The Thermaltake Silent Boost K7 heatsink is used without a fan (since this thing was far from being quiet, at least in my ears...) but the temperatures need to be improved. Read on for the temperatures.
The DC-DC board mounted on the upper right side of the case.
A close-up of the DC-DC board with the ATX cable plugged in.
Close-up of the back of the case to show where the DC input connector leading from the power brick to the DC-DC board has been mounted.
The DC-DC board seemed to handle the system fine and it had absolutely no fans and was almost inaudible except for the seek noises of the hard drive. I measured the following power draw:
Idle, HLT enabled: ~37W
Idle, HLT disabled: ~54W
CPU Load: ~60-70W
CPU + GPU Load: ~65-75W
* But unfortunately, the DC-DC board died just after a few minutes of operation and I'm now in the process of returning it. Therefore, I am currently using my old 350W ATX PSU with two fans (but they're surprisingly quiet). The PSU is run externally, placed on top of the case with the cables routed through the back of the case. So the temperatures I have reached should be comparable with these I'd reach with the Morex 120W since there was no airflow in the case and it was closed.
For a comparison, these are the power draws with the 350W PSU:
Idle, HLT enabled: ~44W
Idle, HLT disabled: ~61W
CPU Load: ~65-70W
CPU + GPU Load: ~78-85W
So there's a difference. One important aspect is to enable the HLT function because it significantly lowers the power draw and CPU temperature.
Now let's talk about something to be improved – the temperatures. They were measured at ~15°C ambient temp (the computer's located in the cellar).
Idle: 30-40°C (after sitting idly for about 30mins)
Load (CPUBurn run for 30mins):
It's clear that these temperatures need to be brought down. The CPU is rated for up to 95°C though, but I'd anyway like to see some cooler temperatures. Therefore, next step is to buy another motherboard that doesn't overvolt the CPU and allows futher undervolting. The stupid PCChips board is cooking the poor Geode at 1.4V and it's rated for 1.25V

. I'm sure lowering the Vcore to the default for the Geode and maybe even undervolting it further will make the temps drop significantly.
By the way: The graphics card, a Radeon 9100, was originally equipped with a 40mm fan which was running fast and therefore loud. I just removed it and the card has been stable at stock clocks since. Removing the fan only limited its overclocking abilites but I don't care about that. It's perfectly fine for my needs and I'll soon try to underclock (and undervolt?) it.
The performance of the Geode is not that bad. It does SuperPi 1M in 1min22s. I don't know if this can be compared directly with those CPUs, but IIRC, these values are in the range of a Duron-1300 or so. Windows and all programs I use on this box feel snappy and responsive. The boot time's a bit long though, but that's due to the notebook drive.
I don't know if the Geode's a real overclocker, but at 1.4V (+0.15V compared to the default Vcore), it ran perfectly at ~1.65GHz. The board doesn't go higher, but when I get another board I'll probably test the overclocking abilities a bit. My first concern, however, is reducing heat output and power draw...
Feel free to comment my system.

Say what you like or don't like and post suggestions how to improve it.
Planned improvements/changes:
– RMA the DC-DC board
– replace the 2.5" drive with a ≥160GB 3.5" drive
– replace the motherboard
– install a slow 120mm fan to generally lower the temps
Eventually, I'd like to thank the entire SPCR community for this amazing and informative website about silencing computers. You've given me more than one good advice.

Particularly, I'd like to thank "frostedflakes" – his Turion64 system made me start thinking about doing something like this and now it's about to be finished.

It's still not as cool as his Turion64, I think...
Thanks
lowpowercomputing