This model is one of the hardest models to open. To get the case off, first remove the 4 screws from the back panel. Next, use a screwdriver to pry the back apart from the top. You can then slide the case off. Next, put the computer on the side and remove any NuBus/PCI cards by grasping the card and pulling upwards. If your model is an 8500, you have to remove the processor card right now too. Also don't forget to disconnect the AV module cable if your machine is an 8500 - it's near the PCI slots. Next, put the computer right side up again and disconnect all of the cables from the logic board. To disconnect the huge power cable, press the latch on it. Next, pull out the power button. Also remember to disconnect the LED cable. Press the button above the logic board to release it. Swing it out to the right and if your machine is a Quadra 840av, disconnect the two video cables. Then, take the logic board out.
Follow the steps from the logic board without pulling the board out (don't bother disconnecting the cables either). Then, disconnect the cables from the hard drive, push the clip down and slide it out.
Remove the case as described in the logic board section. Disconnect the 3 cables from the back of the CD-ROM drive. Pull up the clip under the CD-ROM drive and slide it out of the computer.
Remove the top case as described in the logic board section. Disconnect the floppy cable from the back of the floppy drive. Push down on the clip and slide the floppy drive out.
Information provided by Doug Brown.