It's up to you but I would hesitate upgrading. Software developers have pretty much discontinued support for 10.5 and PPCs. Sure, you can run slightly newer but still now outdated applications, but the PPC/Intel line is sharp cutoff. If you want to surf you can run TenFourFox (my preference is not to go with the latest but to stick with 17.0.11 which you unfortunately can't seem to download any longer) which is still being developed for PPC computers. I run it on my G4 and Tiger OS (plus a few other old browser tweaks you will find on this forum) and almost never encounter problem web sites.
I haven't checked recently but used copies of Leopard (10.5) were not cheap. Be very careful you buy a retail version disc and not one of the much cheaper but useless grey discs that will only install on the model computer with which they were supplied. If you're still facing having to spend $100+ on a used Leopard disc you may want to consider putting it to a used Intel Mac which sees more support.