Hi! Is it just an issue of the backlight (do you see a faint image of an Apple when you boot up and shine a flashlight through the Apple symbol from the rear), or is the screen completely dead? I assume it's completely dead. If it does not show an image via the external video, then I agree with your conclusion that it's probably the video card.
If you do see an image that is just not backlit, it could be the screen, the inverter, the inverter cabling, or the board, but no image at all, especially on external video, points to the GPU.
Just curious though, does the machine chime and basically act like it's functioning aside from the video, or is it frozen or otherwise in some abnormal state?