First off, upgrade the easily accessible RAM to the maximum possible (1 GB PC-2700 200-pin SO-DIMM). That'll make a difference in general OS X usage. THEN do a Leopard install.
As for applications:
1. TenFourFox and Leopard Webkit for web browsing. Any other browser on the system besides the stock Safari should be trashed immediately afterward.
http://www.floodgap.com/software/tenfourfox/ and
http://sourceforge.net/projects/leopard-webkit/
2. TenFourFoxBox for frequently visited sites. I can't harp on this enough. This is one piece of software that makes frequently visited sites work amazingly.
http://www.floodgap.com/software/tenfourfox/box/
3. VLC for any media playback. Set it to default for all files and when you insert a DVD. If VLC can't play it without skipping, probably nothing will.
http://get.videolan.org/vlc/2.0.10/macosx/vlc-2.0.10-powerpc.dmg NOTE: this is a download link for the VLC disc image.
4. The Unarchiver. If this can't open a compressed file, nothing will.
http://unarchiver.c3.cx/downloads/TheUnarchiver3.10_legacy.zip NOTE: this will download a zip file. There's a built-in OS X unzipper to get this app for you.
5. Onyx gives you access to bunches of things that can speed up OS X, general cleaning stuff, and more wonderful things. You will need to find the correct version for your system:
http://www.titanium.free.fr/onyx.html
6. Shadowkiller kills the drop shadows and gives a noticeable speed boost in the process.
https://web.archive.org/web/20110113142807/http://www.unsanity.net/shadowkiller-13.dmg This will download the disc image needed. Make sure to have this as a startup item. This even works under El Cap!
7. Microsoft Office for Mac 2004. I know some people will swear by 2008 and others will steer you to freeware, but I am not one of them. There is even a converter to open docx files in 2004, although you may have to dig around for a minute or so to find it. 2004 is the best I have ever used on PowerPC.
8. Odds and ends from Unsanity. Most, if not all, of the download links work from this archive:
https://web.archive.org/web/20110113142807/http://www.unsanity.com/products Some are not free and I have no idea where to find serial numbers. In addition, there is no way to legit buy them as Unsanity closed its doors several years ago.
9. GIMP is the free Photoshop that doesn't feel free:
http://downloads.sourceforge.net/gimponosx/GIMP-2.6.11-Leopard.dmg?use_mirror= It will need X11, which you can install from the Developer tools on the install CD.
10. iLife '09. It works quite well and includes the surprisingly robust iWeb and iDVD. However, you will need to find iMovie 6. However, note that if you decide to stay on 10.4, you will need to grab iLife 08 and iMovie 6.
11. iWork '09. I like Keynote better than PowerPoint, but you can also get some mileage from Pages and Numbers.
