This was my first Mac.... and have grown to love, love, love my Macs (now at 5) and after moving a couple of times, I finally was in a position to make a new home for my 20" iMac (1.8 GHz). I went to turn it on, and it came on momentarily, then just shut itself off... I was devastated until I found the iFixit website and was able to diagnose the problem myself - needed a new "heart" - aka Power Supply.
The directions were amazing! I was able to do this with only my boyfriend lending some strongarm assistance when we were trying to get the Power Supply put back in place... Previous posts were correct in that it takes some prying on the bottom frame of the iMac to get the Power Supply past the set screw that does not come out... The heavy duty Spudger that I ordered proved itself invaluable!
AWESOME place to find parts!!! Highly recommend!