Soooo, boys´n´girls - I got 16.04 booting to a Desktop, now !
I decided to share what I´ve found, so maybe others, who stranded here will at least make 16.04 boot on their PM G5´s with NVIDIA Cards.
"live nouveau.noaccel=1"
did the trick !!!
Type this at the “boot” prompt of the yaboot bootloader and it will bring up a working Desktop GUI after a while. (when installing, you will have to make this Parameter permanent in yaboot.conf)
Maybe there´s a way to re-enable gfx-card acceleration, when 16.04 was installed - don´t know yet …
It´s a shame not to make use of my FX4500 512MB Card, I´d say!