When I installed the ATI Radeon 9800 Pro Mac Edition -128 MB AGP 4x video card in my dual 1.0 GHz MAC, I had to purchase an Apple DVI->ADC adapter for $99.00.
Getting Motion to run on my G4 was an expensive proposition. My second monitor is still a 21″ CRT so the VGA port works fine for me.
I don’t know if there is such a converter from VGA to ADC since the LCD display requires 28 volts from the video card and that is what the converter adds.
With one and only one video card that can run Motion of a 4x AGP – G4, there arent a lot of choices.