You may or may not want to do a fresh install of Leopard (disk images available at macintoshgarden.org) - as you never know what garbage the existing install is holding - plus you can be selective about what not to install ie languages, printers drivers etc.
Onyx is an essential tool for speeding up the GUI and doing system deep maintenance - also, the app ShadowKiller in removing the drop shadows from windows speeds the interface up.
TenFourFox is the most compatible browser but it's also incredibly slow - if you use it note it cannot use Flash but on the plus side you can install add ons.
Leopard Webkit is an updated version of Safari - it has it's faults but it's much faster than TFF and can still use Flash (search this forum for the Flash hack).
Bear in mind, most websites use masses of Javascript now and this really slows down a PowerPC Mac - you can ease this to some extent with the NoScript add on in TFF - on a site by site basis you can decide which scripts to block or run.
On Webkit/Leopard from the Develop menu you can turn Javascript on/off - you will quickly see which sites can work without it.
Go to
http://winhelp2002.mvps.org/hosts.htm and learn how to change your hosts file - this will stop many advertising pop-ups, ads, banners etc from appearing in your browser.
The quickest way to play Youtube (playing as Flash or HTML5 is incredibly slow/jerky) is with the ClickToPlugin extension in Leopard Webkit - it will open the video in Quicktime or allow you to download it in various resolutions and formats.
If using TFF, there are various Youtube downloader add ons - I use Greasymonkey combined with Viewtube. Some favour the Quicktime Enabler add on - I've found that only works for 1 in 20 videos.
PPC Media centre is essential to download and play videos from lots of alternative streaming sites.
Coreplayer is the most efficient video player for PowerPC - it will comfortably play 720P on your Powerbook and some 1080P with tweaking.
VLC is the most versatile and will play 720P with tweaking.
Avoid Quicktime for video - it's a CPU hog.
I would advise you to decide what you want to do with your Powerbook then search this forum for advice or ask directly - you will always get help.