In the midst of a heated game of Wow-Wow-Wubsy online, my kids' old 20" Intel iMac bought the farm. I considered a new one or a Mac Mini, but then thought, over my wife's objections, "I'm no idiot...I can fix it." The unit simply died and would not power up. Impressively, I diagnosed a dead power supply. One trip to iFixit, and I was committed like a hog to slaughter.
Exactly...I mean EXACTLY like the iFixit iPad App showed me. Brilliant! I had only one leftover screw at the end of the project (a record for me).
Buy the right tools. I really needed a magnetic-tipped torx (star-bit) driver. I worked around it, but it was the only sketchy moment of the whole job (the possibility of dropping a tiny screw into the forbidding innards of my newly re-assembled iMac).