It probly wants AGP video. Knowing how apple can be a pain some times I bet they made the ROM to use the AGP as the primary bus for video instead of it scaning though the buses. If you can get a hold of a AGP video card then you can make sure it's not a dead logicboard. If it keeps doing the freak out when you use a AGP then you might have a bad logicboard.
To live is to let die