I'm sorta in the same boat Silent, I'd love to have a completely passive / silent system 😀

I can't really help you about the CPU questions, I have a P200 in my Dell that just has a passive heatsink on it, but it DOES have a 80mm case fan right next to it. I'm guessing that's why Dell might not have put a fan on the heatsink.

Then on the other hand I have a P166 that has a fan on the heatsink.

What kind of PSU do you have Silent? That's where I'm thinking the majority of the noise in a old box will come from.

Here's what I'm thinking of building: A mini/mid tower with a passive PSU, I won't mind if the CPU has a fan on it. And then to make sure everything is still cool, get 1 or 2 120mm fans in a push/pull configuration and RPM mod them as needed.

If I could cut down the sound on my current configuration by even 50% I'd be a happy camper.

(My main PC box is a Antec P180 w/ all kinds of aftermarket CPU/GPU/North Bridge heat sinks and 120mm fans btw. I've got it running pretty much SILENTLY... and it's glorious running WoW w/o noise 😀 )

Well, at the moment I do some experiments with slot cpu boards.
So in fact I have two cases, one is an old Siemens PCD-workstation with passive ISA-backplane that could hold full size ISA-cards and has a 120W PSU with a separate Papst-fan I allready changed.
The other one is an Acrosser half-size industrial case, it has a 40W PSU with no fan at all:
http://www.acrosser.com/Product/Full%20or%20H … sis_ipc3sp.html

As you can see the problem is not just the noise, it's also the space, as the whole construction gets too high with an additional fan.

Silent Loon wrote:

I wonder if a slowed down Pentium (i.e. Pentium 200) would stay cooler running at 75mhz than the original P75?

I don't think it would make much of a difference, even if you installed one of the mobile versions (2.9V core voltage instead of 3.3V).
MMX-Pentiums also run at 2.9V (2.4V for mobile versions), but they get hotter than the older ones because of the higher transistor count.

Silent Loon wrote:

Second I would like to know if there are Socket 5 / 7 cpus that allready have locked multipliers, so you can't slow them down (I remember that I couldn't change the speed of my P233MMX).

Many P5 models do not support the full range of multipliers - usually they'll ignore the higher ones, lower should work OK.

Generally, it should be possible to cool any CPU up to 100 MHz with a decently sized passive heatsink (typical heat dissipation of all these CPUs is around 10 watts maximum).
However, I'm afraid the space between two adjacent slots on an ISA backplane is too cramped for this. Maybe Heatpipes would be a solution...