My understanding is that while Full HD playback is possible, it's really only doable on Quadcore G5s.
That said, this may be worth a look.
https://forums.macrumors.com/threads/1836256/
It uses MPlayer in the background (though the script) and you get full screen non-stuttering playback.
I use Quicksilver, a quick launch tool, and have created a trigger for the script. All I need to do is select a movie and use a keyboard shortcut. It's the same as having dropped that movie on the script.
P.S. When are we going to see pictures with the recent upgrades you made?
This thread is all about your DA G4 you know!