The slot on the side of he PSU that tightens up the plate from the bottom screw is in a different position than the one from the 20". I have noticed a reduced brightness on the screen when testing out the 17" PSUs on the 20". The 17" PSU can probably be used at this point in the life of your machine, you will need to break off that bottom middle screw or the frame will bulge out when you try to put it in. Your best bet is to shut all power off to the machine via a switch on a power strip to stop any power going to the machine when off. This will help extend the life of your PSU although it will mean slightly longer startup times.