Video card probably. Can it Safe Boot? Shift key down at startup.
The video card isn't expensive but it's the last item on the take apart list.
Like Michael said, some models had the bad capacitors. Take the back off and see if any caps are bulging or leaking, but I don't think that would affect video, it just kills the machine.
Been a little while since I did this but after you have it mounted via FW Target Disk mode go to preferences and Start Up Disks and select the one on the "bad" machine and restart.
John Rougeux Member # -1
Registered: 11/06/08
Posts: 6095
Loc: Louisville, KY
No, can't start up in Safe mode, I did that earlier and it only goes to the blue screen.
NG4, there is no option to go to preferences or anything in FW target disk mode. It only displayed the firewire symbol and then the HD appears on my iMac.