Hello, I'm looking into getting possibly a G4 either the dual 450 or 500mhz model. I know with motion you must have a 867mhz processor in order to install + the graphics card. But would it be possible since 2x 450 = 900mhz? If you can answer thanks alot.

I doubt Motion would install as it requires a 4x AGP slot and the models you listed are only 2x and even 500MHz x2 is way off the 867MHz required. Whether you could do some hack to get it to install or not probably won't matter because it would run really slow.

probably not

go to apple.com/motion and download the compatibility checker