I doubt you would enjoy (at least more recent desktop versions) Linux with that one unless you want to stick with text mode. The slowest machine we have is a Dell laptop which is something like a Celeron 800/256Mb Ram. I find it too slow for OpenSuse 10.3 + KDE desktop.

Other than that machine which I'm not using at the moment, the slowest we have is an Athlon XP 2000 running the above linux set up. My parents are quite happy using that one and seem to generally have a happier time than they did with its previous OS - Win 2000.

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I have actually got Linux running on something less capable, a foxboard. Currently it's just waking up the mythtv master PC in time for a recording but it should be doing other things by the end of the year.