IIvx, IIvi, Performa 600, Quadra/Centris 650, Power Mac 7100
Logic Board
Opening this computer is easy, but it's hard. Some of the parts don't like to come out that easily. Remove the big screw from the back of the computer, at the top of the back panel. Slide the back panel away from you, then lift it up. If your model has a weird plastic piece on top the of the power supply, squeeze the two little levers together and push it off. Remove the other screw on the back that was just below the first one. Lift the power supply out. I found a good way to do this was to remove the two screws that hold the drive carrier in and tilt the drive carrier up, it pried the power supply up. Disconnect all of the cables from the motherboard (power, floppy, SCSI, CD-Audio cable). Remove the two screws on the drive carrier and tilt it up and out. If you want to remove the logic board, simply remove the reset/interrupt switch, and then remove the screw holding the logic board in, and finally, push the levers and slide the logic board out.
Hard Drive
Follow the steps from the logic board until you get to the removing the drive carrier part. Remove it, then take the hard drive.
CD-ROM Drive
Do everything in the above step, but remove the CD-ROM drive.
Floppy Drive
Use the same steps as the hard drive, but pull the floppy drive out instead of the hard drive.
Information provided by Doug Brown.