I've resurrected two G3 iBooks this week by shimming under the GPU, but I have a G4 iBook that also won't boot. I've put pressure under the logic board below where the GPU sits to squeeze it into the heatsink, but it still won't boot. All it does is run the fan.
Perhaps the GPU isn't really the problem in the first place.
I stupidly bought a 1.2GHz logic board on Ebay thinking it would fit in my 1.0GHz iBook. I now know better of course, but I'd rather not buy another logic board if I can get this one kicking.