I've got an old 700Mhz Celeron (P3 ERA) MATX system I'm looking at repurposing as the home router. The CPU fan is fairly quiet but I'm looking at going completely passive so any suggestions on that are welcome. On the power front, the PSU only outputs a max of 100w, so a 120w Pico should be plenty as I'm pulling the burner (will retain the floppy and Zip Drives along with an 33/66 IDE drive (yes it's that old - Runs WinME)

Cooling an old P3 passively should be fine, assuming you can find something big enough that will mount. The TDP of that old chip is only 18W. It may be easier just to rig up a modern quiet fan to the existing heat sink.

However using a Pico PSU is a bad idea. Old computers put most of their draw on the 5 V and 3.3 V lines, which a pico has only very limited capacity. With no cooling the Pico 120 is only rated for 40W combined on the 5 V and 3.3 V, in reality probably less.

You are going to want to get rid of that WinME install if you want to use it as a router. I recommend FreeBSD or OpenBSD for that purpose, but Linux works well.

What is your goal for the router? If you are not going to do anything special, why not get a purpose built device? It will use way less power and would be cheaper than buying the Pico.

The heatsinks that came with the coppermines were very small and lightweight. I've ran several coppermine based systems without fans on the heatsinks, but I always used the ones that came with my old socket A chips because they were much larger and heavier. They use the same mounting system so it's not a big deal to throw one on there if you have one lying around.

When you say completely passive are you referring to the entire system? I would think that you'd still need at least a little airflow through the case. If it's an older case most of the ones from that era were designed for 80mm fans. Maybe you'd be better off going with a quiet power supply with a 120mm fan instead of the pico? I've ran several coppermine systems where the only fan was in the power supply. Never tried to run one completely passive though.

What will you be running the OS from? If you're planning to use an ancient hard drive just keep in mind some of them are pretty loud. In builds using old hardware I've usually found the whine of the old hard drive to be the loudest most annoying sound coming from the system. If the OS you're planning to run is pretty small you might consider an IDE to CF adapter or SD card adapter. I doubt your board would boot from a USB thumb drive. Of course you could go with a SATA PCI card and a SSD, but that seems a bit pricey for what you're trying to accomplish. Eliminating the mechanical hard drive would also reduce the amount of heat generated inside the case it make it even easier to cool.

As washu said, if all you're looking for is a basic router then just buying one is going to probably be cheaper and use way less power.

Washu:
Thanks for the comments about the possible issue with the Pico PSU in regards to the 5/3.3 volt lines. If needed, I can salvage parts from a dead AT PSU to get the needed connectors and simply build my own version of a Pico. The main reason I thought of the Pico is that the OEM PSU is pushing 15yrs, so I'm not to hopeful it'll last much longer. On the power saving from a new consumer router, hate to tell you, they really aren't anymore efficient (12v/5a=60w) as the rating on my current Netgear unit shows. I'm looking at 80w max load unless I'm using the Onboard graphics, which can push system load to 85-90 watts though I suspect it's closer to the 60 watts of new routers - Reserve power for additional hardware. On the WinME install - that was simply to provide a timeframe for the age of the system as it certainly isn't going to run it.

Blade47:
Thanks for the comment about the Socket A Heatsinks - I've actually got a monster that was used with an XP1800 that could handle it passively. On the passive cooling, I'm looking at a completely silent unit, just like the blasted netgear is. Ventilation isn't a problem as I'm building a new case for it.

Small routers definitely do not consume 60W! Not even close...

To put power consumption into perspective, the DN2800MT is a low-power x86 board which doesn't need a power supply and idles around 10W. It's on the expensive side but you've got to take into account the price of the pico.
If you want to hack something together, consider getting a damaged low-power laptop from someone wants to get rid of it. Some low-power laptops consume even less than the DN2800MT.
And if you're OK with an ARM instead of an x86, consider running your own software on a used small router (only a few models allow this with minimal effort so you need to do some research before getting the unit) or procuring a Raspberry Pi (a USB-powered board).

Any of these solutions would be affordable and considerably more power-efficient than your P3.
There are also low-power board which have two ethernet ports on board but the most common models are less efficient and require a power supply.

The ratings on a unit aren't going to tell you much about the actual power consumption. A consumer router isn't going to pull anywhere near as much power as an old P3 system. If you have a killa-watt or something similar lying around you can test it yourself. I've tested several routers in the past and can't recall the exact wattage, but I know they were all less than 5 watts from the wall. All of my old coppermine based systems pulled anywhere from 20-43 watts at idle depending on the motherboard, add-in cards, hard drives, whether or not it was undervolted, etc...... I don't recall exactly what the lowest wattage was, but I do recall it was in the 20's. These were stripped down systems running as a NAS box with 1 drive for testing purposes.

The only way I'd consider using an old system instead of a consumer grade router was if it offered something the off the shelf router didn't. Until recently I was running Untangle, which did far more than my router, but it requires something beefier than a P3.

HFat beat me to it, but he is correct that no consumer router uses anywhere close to 60 W. My guess is that your power brick is actually 12 V at 0.5 A, which is 6 W.

Again, unless you need the flexibility of a full PC then a purpose built router is a better choice from both a cost and power perspective. You also save a bit of power if you don't need an additional switch.

I do use an older Atom board (D510) as my router as I do need the flexibility and I run other services on it. It idles around 22 W, and that is with two 2.5" drives and an extra dual port NIC.

I did use an old P3 as a router for many years before Atoms were available. With a single 2.5" drive and extra NIC it idled around 45 W, which was mainly because it was a Dell business class PC. For all their faults Dell usually has pretty efficiently power supplies. With a Antec power supply of similar age it idled at 65 W.

Also, a PC of that age is going to have questionable reliability at best. While P3s were mostly before the big wave of bad capacitors, time will still take it's toll. I wouldn't want to use such a PC for anything that might need reliability or data integrity, both of which a router most certainly does need.

Blade47:

You didn't catch the 512M and IPv6 issue. I doubt that any $40 dollar consumer router is going to be a match for the performance. In fact, you'd have to be 3-5x the cost and looking at a business class router to even get close to what the repurposed system will provide. Yes it's going to cost a bit to add the needed 4-8 port card but that's the only thing I need to buy for it as I've already got a WiFi card (b/g) and single port (WAN) card with full support in the Linux kernel. No binary blobs or anything needed. The only issue is how much the multiport card is going to cost as I want a spare as I have at least 2 working spares of the single port NIC and the WiFi cards (never throw working hardware away).

It is a Slot 1 or Socket 370 based Celeron? This makes a big difference for how you can cool it.

I have an old Slot 1 Pentium 3 550 where I replaced the heatsink with one modified from an Athlon 700. A heatsink for a 45W CPU on a 35W CPU means that I run it with no fan. Instead it just depends upon the intake fan on the PSU just above it:
viewtopic.php?t=51604

Alternatively I would look at using a bigger than stock fan and just screwing it on where you can. I have also done this in a dual processor 1GHz Pentium 3 system. Each CPU has a separate Socket 370 heatsink and the 2 50mm fans have been replaced by a single 140mm fan running at 1000rpm!

Seems you're set against it, but today's Newegg email deals included this Asus DD-WRT capable router / wireles AP:

http://www.newegg.com/Product/Product.a ... 6833320076

ASUS RT-N12/B1 SuperSpeedN Wireless Router with 2x5dBi Antenna / Open Source DDWRT Support

Reg price: $49.99
Your Price: $34.99
With Promo Code: EMCNDHD223

$35!

http://www.dd-wrt.com/wiki/index.php/IPv6

A cheap option for playing around with DDWRT...