Additional comments to fregac's write up for a 17" iMac G5 first gen.:
1. It helps to thicken up the foil by crumpling it in straight pieces or oblong shapes to fit in and around the capacitors. On a 17" iMac G5, first generation, there are capacitors extremely close to the GPU, building a thick shield of crumpled foil helps.
2. You will need a T8 star bit (Torx?) to get the heat sink off on a 17" iMac G5 (maybe others too). I could not get my assortment of allen wrenches to work.
3. Use a spudger or tiny plastic blade to scrape off old thermal grease.
4. The GPU is the chip labeled "nVidia GeForce FX".
5. The northbridge is the one with plastic wrap, small capacitors, and a few IC's on it. It's on the same side as the GPU.
6. The PPC CPU is on the backside of logic board and labeled "IBM" and "U3"
7. Remove battery for added safety before installing foil (foil is conductive!!)