I use Debian on my PowerBook G4, which is not quite the same machine, not to mention not nearly as powerful, but since it works decently for me it will probably work better for you.
As for flash, its true that Adobe Flash Player doesn't support the PowerPC architecture, but you can use Gnash
instead. It works reasonably most of the time, but my 1.5 GHz G4 and Radeon 9600 is quite underpowered and struggles with most video > 480p (or 720p using mplayer -really-quiet -framedrop -lavdopts skiploopfilter=all
, which I aliased to mplayer-fast
). With your much more powerful machine it will probably work fairly well.