I have to admit I'm rather confused by your question. The Intel iMac has a Terrabyte sized Hard Drive, which I assume you mean (not gigabtye "GB"). The 21.5 inch iMac will come with a 1 TB drive as standard. However, the larger 27 inch iMac will be available with a 1TB, 3TB, 768GB SSD or 3TB fusion drive with 128 gigs of flash ram on board. They'll be single drives, but no-one as yet has any information on the physical dimensions of the drives.
Two main issues may have happened to your [URL=http://www.forevermac.com/category/apple... |iPhone]: 1) A Dead battery, which means that it simply won't even hold the date/ time. There's always a small amount of charge left in the battery that ensures it maintains the date/ time. I suspect this is fed by battery and not the actual charger- hence it won't power on properly from the mains. Replace the battery. 2) The Power Management unit is corrupted/ your OS is corrupted. Far more unlikely. I would replace the battery to see if that fixes it.
Unfortunately it won't see a drive larger than 128 gigs. The hardware is 28 bit and not 48 bit so can't address drives that large. However, according to this page: http://www.forevermac.com/1999/08/g3-blue-and-white-350mhz-macintosh-computer/, a patch can be applied to circumvent the limitations. Not sure if Debian has the patch for 28 bit Chipsets.
An after market third party USB dongle can be used on the iMac G3, assuming the iMac is running MacOS X 10.3 (all models will run this, including the Caddy Loading iMacs). However, no iMac G3 (or iMac G4 for that matter) can use an airport express card. These computers are all Original Airport card only (802.11b). A third party USB dongle will provide Airport speeds equivalent to 802.11g. ref: http://www.forevermac.com/1998/05/imac-g3-233-mhz-dekstop-computer/
The Power Macintosh G4 MDD should have a PMU (CUDA) switch to the mobo that you need to depress. I suspect that the 1.8gHz caused settings to be changed in the Mac. You can always do a pram reset or pull all power for half an hour (including battery). You could start up from open firmware (cmd option o and f) on power on and once in there (grey screen with some text), type reset: nvram, hit return and it should say: Ok and then give another command prompt, then type reset: all and hit return again. Mac should then re-boot.