If your needs are that simple:
My Ubuntu 12.04 remix will fly on that machine. Its actually what I use on my G5 (with void on another partition for testing purposes). Super easy install. The dvd boots to a live desktop to test before installing.
Updated to v2 (03/04/20) Here it is. New and improved over the stock 16.04 release. You can use as a live dvd / recovery system, or install it. What's changed since the original release back in 2016? Added SpiderWeb, SpiderMail, InterWeb and Arctic Fox (modern web browsers and email client)...
forums.macrumors.com
Linked near the bottom of first post.
Otherwise there's still Ubuntu 16.04, Adelie, Void-PPC, MintPPC (debian based), Fienix (debian based), Crux-ppc, and Gentoo. There's also the BSD's but they have crap for browsers.
As for youtube....
I came across this, and decided to give it a shot on Ubuntu 16.04.7 (remix). I'd assume it'd be possible to build the CLI version on 10.4/10.5 as well with macports perl, but i haven't tried that yet. It works very well. Fast too. Download the 0.0.8 release found here. Now you'll need to...
forums.macrumors.com
...and updates for the included browsers with a few extra browsers added: (scroll the whole page for more)
Web Browser for PowerPC Linux and Mac OS X 10.6 and above - wicknix/SpiderWeb
github.com
Cheers