I finally got around to installing YDL 6 on the G5 Quad. I wasn't able to install using the GUI, so I install-texted my way through it and made it a point to Xautoconfig once I was up and running.
Sure enough startx didn't do much, so I Xautoconfiged. Nothing. Xautoconfig --safe didn't do it either, so I tweaked /etc/X11/xorg.conf to display in 16 bit. Then 15 bit. Then 8 bit. Then using VGA. Then I gave up and dropped back into OS X.
How do I get X to play nice with a GeForce 6600?