It is possible, at least for a lightly loaded cheap(ish) office/net PC. And for a lot less money than a TNN500. I finally managed to achieve silent nirvana yesterday. Here's what you'll need:
Any cheap ATX case
Antec Phantom PSU
Asus K8V or A8V (with Asus Qfan technology and a passive Northbridge)
A64 2800+ Newcastle (130nm) or Winchester (90nm)
Zalman CNPS7000B-Cu (or AlCu) HSF (or possibly XP-120)
Fanless GFX, such as ATI 9200
SilentMaxx HD Enclosure or (imported) Smart Drive 2002C
Samsung Spinpoint 1614N 160GB PATA
Single optical drive
Mount the Phantom externally, on top of your case. Turn it upside down so the larger fins are facing upwards, and the power switch & socket are facing the front. Feed the cables in through the PSU opening. Leave the case door off. Rest the HD enclosure on some foam at the bottom of the case, as far to the back as possible. (It's more noticeable, noisewise, mounted higher up in a drive bay).
Enable Cool & Quiet and Asus Qfan in the BIOS. I flashed back down from 1008 to 1006 to make sure Qfan would stop the Zalman fan spinning. I hadn't noticed 1007 or 1008 doing that lately.
After powering up, wait a couple of minutes for Qfan to kick in and stop the Zalman HSF. CPU temps should be around 37-43C. My system temp is quite high at 40-43C, but that's because of my red hot (even at idle) passive 9800 Pro Ultimate. A cooler GFX card should bring that down a good few degrees.
The Phantom is absolutely silent. I can't hear any "coil buzz", even sat exposed on top the case. It does get very hot though, either inside or outside of the case.
Other motherboard manufacturers might have boards that have something similar to Asus' Qfan technology, but I don't know which ones. Cool & Quiet will not turn your HSF off, only Qfan will do that.
The 130nm Newcastle has a TDP of 89W, so a 90nm Winchester with 67W might be preferable. Although if a recent THG article is to be believed, the actual consumption of a 3500+ at full load is only 31W. (Can SPCR labs verify this? If true, why would anyone bother with a Pentium M?)
A copper Zalman would probably work even better than my aluminium/copper variant, at bleeding the heat off a few degrees cooler. If you're not hefting the PC anywhere (unlikely with the PSU balanced on top), the additional weight shouldn't matter too much.
The Samsung PATA is the only HD I've found which will remain cool inside an enclosure. The SATA drives all seem to run a lot hotter, for some strange reason - even the equivalent Samsung SATA. My PATA sits at around 33C all day. With no fans running, you can just hear a low hum.
Qfan will start spinning the HSF if your CPU gets too hot, then stop it again when it cools down. Or you could use SpeedFan set to 30%; you can't hear the fan at a couple hundred RPM. It's unlikely the HSF will start if you're just typing documents or browsing the web.
For the future, I've got my eye on an Asus A8N-E Premium nForce Ultra, A64 3500+ 90nm, ATI X800XL PCI Express with the new Zalman AlCu/Cu GPU HSF, and a bay-mounted fan controller. When I'm playing games, I can switch the rear 120mm Nexus on, and possibly the front 120mm as well in summer if necessary. While idling in XP or listening to music, it should be possible to have all the fans off. Bliss.
How To Build A Fanless Stock PC
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