Faulty GPU or Logic Board?

Hi,

i have an issue that i do not understand. After a Lightning my imac stopped working. I can power on and ear chimes but no screen , neither with an external monitor. Thinking the video card was broken, i have cooke my GPU in the oven (The gpu is an Ati HD 2600 Pro). I did this successfully a lot of time before but this time did not work. Now if i attach my GPU on the Logic Board The three leds are flashing but no boot at all. No chimes, no fans ecc... The strange think is that if i remove the GPU from my Imac it boot! I can ear chimes, hard drive, optical and fans. O_O I would like to know if the problem is the logic or the gpu. If is the gpu, can i upgrade with an 8800 GS from another imac ???

Sorry for my bad english and thanks to everybody that would like to help me ! :)

Best Regards from Italy, Alessandro.