I'm a quiet PC enthusiast. This also does concern my retro builds. My definition of "quiet" is: "If I cannot tell if the computer is on or off untill I put my ear to it, then it's quiet".
Let me share with you how I built a quiet retro PC around 3dfx Voodoo 5500.
It is a Socket A system and draws a lot of power. I tried putting a BeQuiet PSU inside it, and it gets pretty hearable after 30 minutes of playing. It has 30A on 5V line, should be enough for my Athlon XP 2000+ and the Voodoo, but since the power usage is >50% on 5V line, it needs extra cooling.
So I decided to mod one of my old PSUs, it is FSP Group which also has 30A on 5V line. I've put a Noctua Redux R8-1800 fan on it and it was loud as hell! And since I just drill a hole in the PSU and connected the fan to motherboard, there's no way of knowing whether the cooling is adequate enough.

I've bought a fan controller STW-6041, which has 4 temperature sensors and 4 controllers, which fit well enough for my build.
My 3dfx Voodoo 5 was already modded: I've put 20mm Noctuas on it and connected them via LNA adaptors to PSU's Molex (to those who don't know, they are Low-Noise adaptors, basically it's just a resistor, which reduces voltage from 12V to 7V to reduce fan's RPM.

I put 1 temperature sensor onto the hottest point of the PSU, right when its case touches one of its heatsinks. Enough to know how good or bad temperatures are, 2nd sensor onto a north bridge chip (because why not?).

Now for the fun part. 2 remaining temperature sensors went to the hottest measurable parts of Voodoo 5 - the backs of its graphics chips. The sensors are glued to the back of the card with remnants of thermal pads from Accelero Twin Turbo II, which I've installed on my girlfriend's working machine.

I fired the system up and in 10 minutes of running Unreal demo they both heated up to 60C. Now it was apparent to me why the system shut itself off before. It's just plain and simple overheating and I had no means to check the temperatures!
What I did next is I put 2 small aluminium heatsinks on top of the thermal pads while the system was running. To my astonishment, the temperature decreased immediately from 60C to 53C!

Since these are very small heatsinks and the thermal pads are way too thick, this didn't do much and the contraption didn't look very stable to be honest.
So I bought bigger heatsinks and 1mm thermal pads at a hardware store. Here is the final result:

I've played a couple of hours on this system, with video card's fans' speed reduced to 3000RPM (4500 is max and kinda loud for me).It looks nice and stable and the temps didn't raise above 53C.

I didn't put the sensors directly under the heatsinks this time, so the readings might be not as precise, but putting the sensors between the heating surface and thermal pads is not a good idea, because it reduces the actual cooling.
I know that some people use freezer spray to remove stock Voodoo 5 heatsinks and reapply thermal glue, but I'm wary of doing that, because the video card is not cheap to replace and I'm not sure I won't damage it in the process by accident. Besides, I don't have heatsinks that I'm sure will work better than stock ones, so I'll just leave them intact.