Someone gave me a G4 [733mhz, 768MB RAM, Geforce2 MX 400] with OSX 10.4.8 installed. Also has Firefox 1.5 and Flash 9. I have tried a few videos on Youtube and they seem to play ok.
I have installed Debian 5.04 on the same G4. Performance, using XFCE4, is a little slow compared to OSX. This could be partly due to the only Nvidia driver available for PowerPC is the xorg nvidia driver.
I have installed Firefox 3.6 which seems to run ok, however, there doesn't really seem to be a Adobe Flash plug-in available for Linux on the PowerPC platform. I found this link
telling me about alternatives such as Gnash and swfdec. I tried both. swfdec didn't even seem to be recognized by Firefox. Gnash kinda works on some sites with flash images, but all I have for Youtube is a empty black box. It's possible newer versions found in Debian testing or unstable may work better...
I think I could get better performance out of Linux on my old AMD K6III+@550mhz than I can get out of this G4 running Linux.
I'm finding out that support for Linux on PPC isn't as great compared to the x86/x86_64 platforms.