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I have been playing with my wife's old computer since she upgraded to a new Dell laptop. It is a 1.2ghz Celeron with a nVidia TNT2 graphics card and 256MB of RAM.
Arch Linux runs great on the old machine. Configuration is minimal and once I got my head around Pacman, I really liked it. The Arch Linux wiki is really useful, by the way; reading it helped me figure out how to install the nVidia drivers for my graphics card.
BTW, this old rig runs BeOS and XP as well. You can check out screenshots on my Flickr page (all since 2006-05-19 are on the old computer):