I have 3 old dinosaurs that are, were, or will be running GNU/Linux. My very first dedicated GNU/Linux box bought off of eBay for $20, is a Compaq Deskpro 4000 with a Pentium Pro 180MHz, 96MB of RAM and an old Western Digital 4GB hard drive. I've installed Apache 1.3 and it works great for serving. It even has X on it and when setup properly nicely runs fine.
The other box is still needing parts, but it's another Compaq - but a Proliant rack server with dual Pentium Pro 200MHz CPU's and it came with 512MB of RAM (iirc) along with two Seagate Hawk ST15230N Fast-SCSI-II drives connected to a Adaptec AHA-2940AU 32-bit PCI to Ultra SCSI adapter. Interestingly enough, it was apparently in eBay's care because no one wiped the hard drives. I need two processor power modules for it and I'm set.
The third is an old custom job I bought at a local church garage sale for $15 iirc. It's an AMD K6-2 500 with 512MB of RAM, and a Western Digital 17GB IDE hard drive. I have 8.04LTS on it and it runs a LAMP stack excellently (installed some groupware/portal stuff).
My firewall is a 750MHz AMD Athlon (slot A), 256MB, running Copfilter/URLfilter. Never even touches swap even with using all the features.