All I can do is tell you about my current setup using Mac OS X 10.5.8 Leopard.
I have a PowerBook G4 17" running 1.67Ghz 2GB. Currently I'm using the latest version of Camino web browser.
YouTube videos are choppy, so I use an app called YouView. Which plays back YouTube fine.
I run Skype for Leopard and that work without fault. iTunes 10 seems to allow me to download and play video from the iTunes Store with ease. I can't sync my iPhone on IOS 6 due to Apples change in system requirements for iOS6 being Mac OS X 10.6.8 and above.
Can't comment on Spottily you will have to see what version are available. With exception to my iPhone, all works fine, even some flash sites seem to work ok on Camino web browser.
Wouldn't use safari as that's out of date on 10.5.8, but found no other software restrictions yet that won't load or install on Mac OS X 10.5.8
I admit I run out of date software like photoshop CS 3 as it supports PPC, granted its not as good as PS CS 5 but it does the job. I run other out of date software like TeamViewer, but at the moment it seems to connect fine to more updated machines running TeamViewer. I guess it will depend on what you want to do with your PowerBook.
If its just web surfing then I would suggest Camino it's like Firefox, it's up to date on this platform and work just as we'll as that of newer macs.
Best of luck
I suppose one factor is whether I can get hold of a Leopard installation DVD without breaking the bank, any suggestions?
That's the challenge, as Apple no longer sells 10.5 Leopard. One source for unopened Apple software is hardcoremac.com. If you go to the used market, avoid any gray-faced Leopard disks, as they would have been delivered for other systems and are locked to those systems. Leopard was never delivered on Powerbooks.