Even the cooler 68k and early PPC machines still sell for more than you would expect.
I sold my Rev.A 1.6HGz 17" G5 iMac late last year for £450. Quite mad, really. When I come to sell my 24" iMac and 1Ghz TiBook later this year, I hope to get quite a lot back too.
Wow, XP was out that long ago? I got my first computer (that was really my own) in early 2003. Before that my aunt gave me her old computer with Windows 98.