ExtremeTech has an article about building a home Linux server. They’re recommending high-end P4s for the task. And I say, get real.
If what you want is a simple file/print server, anything that’ll take a 100-megabit NIC and has room for some good-sized hard drives will do great. You want a machine that’s running its PCI bus at 33 MHz, so a Pentium-133 is a better server than a Pentium-120, or, believe it or not, a Pentium-150. If the machine is marginal, get something other than an $8 D-Link 10/100 card or another card with the RealTek 8139 chipset. A pricier 3Com or Intel card will conserve CPU cycles for you.

Remember, too, that Linux doesn’t use the BIOS, so if a machine refuses to recognize that 200-gig hard drive you just bought, set the drive type to “none” in the BIOS and keep another, smaller drive in the system to boot from. Linux will pick up the monster drive and use it.

SCSI is much better for servers than IDE, but when two or three people (or one person) will be using it, the only advantage SCSI really offers is being better-built.

And the video recommendations in the article are absolutely ridiculous. You don’t need a GeForce 4MX 420. Dig around in your parts closet and find that 1-meg PCI video card you bought back in 1995 and haven’t used in five years. We’re talking a system that’s going to be using text mode. Or buy the very cheapest OEM AGP video card you can find to save a PCI slot for something useful–last time I looked, Newegg.com had a cheap AGP card based on an old ATI chipset for 18 bucks.

So don’t listen to those guys. If you want to build a Linux server and all you’ve got to work with is a Pentium-100, go for it. It won’t perform like their aging 1.13 GHz P3 (the slower machine in their benchmarks) but for a home network, it’s plenty. Keep in mind this Website is running off a P2-450. I’ve watched it under heavy traffic. There are two bottlenecks when it’s serving files to someone on broadband: My DSL connection, and the Web browser on the other side. The only time I’ve ever seen CPU usage on this box top 50% for more than a few seconds is when someone loads that giant GPS thread (the post with more than 200 comments).

Just be aware that some Linux distros aren’t too wild about older BIOSes. I’ve got a P133 that won’t boot the Mandrake 7.2 CD (yeah, it’s old–that’s how long it’s been since I used Mandrake) or the Debian 3.0 CD, but Debian 2.2 works fine. So be aware that you might have to experiment a little.