iMac 2007 and 2008 Ati HD2600pro GPU bios is not recognized by Radeon Bios Editor so i’ts not possible to undervolt it; to prevent it from failing after a successful reflow, we tried to improve it’s thermals with a couple of modifications. We used thermal adehesive and metal wires to attach small heatsinks on the front of the cooler. We had to adjust the position of the sinks based on the shape of the iMac chassis, and we used thermal pads to have them contact to it acting a bit like a heatsink.
The second and probably most important mod is blocking the HDD fan original flow and cutting a hole to route it to the GPU heatsinks we just installed.
I am not actually convinced the one on the back is beneficial, as the screen could actually give more heat than it is taking, and was also the hardest heatsink to properly mod (the picture was a WIP, ended up very flat). Still, it could be that it’s the thermal shocks (rapid changes of temperature) that cause the gpus to die and not actually the heat itself, so the more “connected” the whole machine is (acting as a huge heat buffer in either direction), the slower the temperatures change.
May your iMac gpu never fail again! Remember that on old iMacs the screen is the main power draw and heat source, so keep the lights in the room and the backlight on the screen as low as possible!