Upgrading your Computer

Every time I hear you can't upgrade a Mac, I think about the old G4 that had 3 new video cards, 2 new hard drives, a new DVD-RW drive and extra RAM... and the laptop with more ram, a new wireless card and a larger hard drive... and the overclocked beige G3 with 2 installed video cards, extra RAM, a SCSI card, and a USB/IEEE1394 card. For me "can't upgrade" means "it's full."