When I checked what is installed : there is no more video card!
I have exactly the same problem since A month or so. Mine is a 3.06 GHz with a NVidia Geforce 8800 GS. My guess is NVidia screwed up as they did on the 8600.
I have the chance that the iMac is cnfigured for sharing screen (and actually I do have Logmein installed). So with these means, I can see the computer working from another one.
Computer is running fine. When I checked what is installed : there is no more video card !
Tried SMC, PRam, praying a lot; nothing worked. I suggest we work together on this. I heard by overheating the Geforce for 2-3 minutes to 500° Celsius, it could restore functionning. But that'll be my last option.
Any thoughts ?