Unfortunately what you need is something that is designed for PowerPC Macs. Stuff that is designed for Intel Macs will not work. The PowerPC (PPC) Mac software is getting scarcer. Macs in 2006 shifted to Intel CPUs. Stuff that is designated Universal will work, and you can buy the retail 10.5 if you have no Mac OS 9 related software that needs to load. If you partition your hard disk, or use an external Firewire hard drive partition just for booting into 10.4.11 or earlier, you can still use Classic for Mac OS 9. My user tip here discusses your options:
Don't dump your PowerPC Mac yet, Appleworks has no equivalent in the Intel Mac world once upgraded to Mac OS X 10.7 or later, and this tip covers Appleworks:
So you are unfortunately stuck in the transition period, but if carefully reviewed can be done right if you ready all your documents for the transition when you can afford it.
'a brody' provides good advice. I also use a 2002 G4 with OSX 10.4.11. Me, I just don't sweat it. 95% of what I want to do my computer still does with older software. Another 4% I can find work-arounds (if somebody sends me a document in a Word format I can't read I tell them to send it to me as .rtf). The last 1% doesn't keep me awake at night. It's only when things happen such as my e-mail provider requiring I use a newer version of a browser that I then have to upgrade. You will find for some select software there is legacy support, such as TenFourFox which is a newer browser for PPC Macs.