Don't forget we are talking about an imac here. So the hardware options are limited. For my 2 I would either buy a less expensive 120gig drive or use an external firewire (assuming the imac has it) enclosure. I would not spend the money on disk space I cannot use.
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...