Pretty simple. Just tell us the specs of the oldest Linux machine that you own. I'll start.
Mine is a Dell Dimension 4500. Pentium 4 2Ghz 773MB DDR2 RAM ATi Rage 128 32mb video card Belkin F7D1101 v1 Wireless USB Adapter 80GB HDD
All this runs Linux Mint Debian Edition with the XFCE desktop. This thing is pretty fast, as long as I don't put it under much load. It's good for web browsing with Iceweasel. It even performs this function better than it did when it ran XP. I also use it to route my wireless internet to my 360 via an ethernet cable.