First of all, lift the latch on the left side and pull the side panel down. The logic board is under the side panel. You need to remove the FireWire and the modem. To remove the FireWire card, remove the two screws that hold it in place, disconnect the cable that goes from it to the logic board, and remove it. Do the same thing with the modem. It is also held in with two screws and a cable goes from it to the logic board. Don't forget to remove the PCI cards. Remove the screws that hold the motherboard in place and remove the motherboard by tilting it up and out.
The hard drive is right at your view as well. You can undo the screws on the hard drive plate and take it out if you want. To take it out, it depends on which model you have.
ATA: unplug the power cable and ribbon cable from the logic board.
SCSI: follow the ATA steps but disconnect the ribbon cable from the PCI card.
The computers actually have two different drive carriers. It depends on when you bought your computer. Older ones have a single carrier that holds all three hard drives. Newer ones have three separate carriers. As far as I know, some Blue and White G3s have the single carrier, and some Blue and White G3s and all G4s have the separate carriers.
Unplug the power cables from each hard drive.
Now, there are different sections depending on which drive carrier you have. Look inside and go to the appropriate section.
Single Carrier: Remove the screw from back end of drive carrier. Also remove the metal piece that holds the carrier in. Slide the drive assembly toward the back of the computer until it is released, then lift it out.
Separate Carriers: Remove the screw from the drive carrier you want to remove. It is at the front of the carrier. Tilt the carrier up and remove it. Note: on some models, the carrier on the right has ends that extend higher up than the rest. This is to accommodate a second drive. According to Apple, only the models with this special carrier support dual ATA drives on the hard drive bus.
CD-ROM Drive/DVD-ROM Drive/Zip Drive
To get into this section, open up the case as normal. Next, push the two little tabs that are connected to the front face place (they are hidden behind metal, on the left side if looking at it from the side where it opens). Rotate the faceplace forward. Next, remove the two screws that hold the drive carrier in place, and push it forward a bit. Remove all of the cables, and pull it out the rest of the way.
Information provided by Doug Brown.