My A520 is in a mid tower case from cyberpowerpc. The A520 will fit in the G4, but keeping it cool is hard. If you cut away the shelf leaving just enough to hold the power supply, the ram will clear when the door is closed, but any fan on the cpu will hit the PS. But, with the shelf cut away you can use in AIO water cooler, just not a very big one. And the case has very bad airflow to begin with.
But with all that, I really love the G4 looks and always wanted to do one!