I just solved this issue!!! The problem is, I've no idea how. I've bought a second graphic card on eBay, but even with the 'new' graphic card the problem was exactly the same. So I've unmount the motherboard again and again. I've removed the battery of the motherboard for a few hours. I've tried the 'old' graphic card without cooler: I just plugged it into the slot (without cooler) and without unmounting the motherboard.. and bingo.. the iMac start working again. I've really no idea why. I plugged the graphic card 'the hard way' without a view on the slot. May be it has moved a pin or something ... I really don't know. I was able to remount everything correctly, with cooler and all cables, and the iMac is working again. That's a mystery.