MacOS X 10.2
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MacOS X does not require a SN. MacOS X 10.2 incorporates many extra features over 10.1x.
Is like Win XP and Win XP SP!. I Like MacOS. In terminal mode it is completely Unix. Great stuff. And really, with 2 1GHz G4 procs, its quite fater than a 2 GHz Pentium IV.