whatever you do install the machines grunt wont play nice with the GUI.
I would reccomend fedora as your first and a lightweight version at that without a GUI would be better for the space and machine.
free bsd ad or open bsd are pretty good,
a friend of mine is one of the first in the world to get openbsd running on a se 30 mac
be carefull of distros that need to compile everything from scratch or your literally going to be waiting for weeks for it to compile on a Pentium I
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