Steven J. Vaughan-Nichols has tried to give an answer to the question: “Which Linux should I use if…”. He likes Xandros, but also mentions SimplyMEPIS and Ubuntu.
Read it at desktoplinux.com
I want to run Linux on an older machine.
There are many great lightweight distributions, but I keep coming back to MEPIS's MEPISLite. Weighing in at about 640 MB, MEPISLite will run on systems that were slow when Windows 98 was new.
By using KOffice, instead of OpenOffice.org, and using the optional 2.4.29 kernel, I've managed to get MEPISLite working quite nicely on a bargain-basement system with an 800MHz Via processor, 128MB of RAM, and a 20 GB hard drive.
You can certainly run Linux on even less-powerful systems with distributions like Damn Small Linux, Zenwalk (aka Minislack), or Puppy, but for the best combination of low system demands and features, MEPISLite, and its big brother SimplyMEPIS, is hard to beat.