I have on old mac desktop:
- Power PC G3 (300MHz)
- 320 MB Ram
I want to use it for browsing the internet and playing videos. What distro should I use?
You might check out Crux, it seems to be the most promising entry in lightweight distros that support ppc catagory.
I also ran across Slackintosh, although their promotional material isn't very exciting. You can find lots more distros that support ppc here. Several distros listed (including Ubuntu and my personal favorite: PLD-Linux) have actually dropped PPC support over the last couple years, so that that list with a huge grain of salt.
I'd say go with Debian. It supports many architectures including ppc.
Debian's quite user friendly and because it has a large user base, you'll probably get any questions you have answered quickly.
Debian has a comprehensive installation manual.
EDIT: I just came across this site which seems to have quite a bit of info for ppc GNU/Linux users.