Thanks for the advice 
I installed a 160 GB drive in the G3 this afternoon, replacing the 3 old small horrible ones that were in there, and it works really well 
The Mac seems even faster now, and it's good to have a drive that things don't randomly go corrupt on! As suspected, only 128 GB of it is available, and I don't want to risk trying the software that was mentioned as a way of avoiding that limit. Used drives seem to go for a good price on eBay, so the old 120 GB drive from the Acer that I was considering using in the G3 should almost pay for the new 160 GB one! I would certainly recommend to anyone else who's using an older Mac with a similar vintage hard drive to upgrade it, as it really does improve your Mac experience!
I used Carbon Copy Cloner to copy across the OS X installation, which seems to work pretty well - it wouldn't boot at first, and booting into 9 and running XPF showed that BootX had disappeared (possibly it's one of the things that CCC removes), and replacing that made it work again
PS I don't suppose anyone knows any good ways of reducing the amount of stuff on OS X's startup disk? (like useless files or caches that can be deleted, such as the remains of downloaded updates or things like that) Just that with 8 GB it seems a real squeeze, and my startup disk seems to get fuller and fuller even though I'm not installing things to it!
I installed a 160 GB drive in the G3 this afternoon, replacing the 3 old small horrible ones that were in there, and it works really well The Mac seems even faster now, and it's good to have a drive that things don't randomly go corrupt on! As suspected, only 128 GB of it is available, and I don't want to risk trying the software that was mentioned as a way of avoiding that limit. Used drives seem to go for a good price on eBay, so the old 120 GB drive from the Acer that I was considering using in the G3 should almost pay for the new 160 GB one! I would certainly recommend to anyone else who's using an older Mac with a similar vintage hard drive to upgrade it, as it really does improve your Mac experience!I used Carbon Copy Cloner to copy across the OS X installation, which seems to work pretty well - it wouldn't boot at first, and booting into 9 and running XPF showed that BootX had disappeared (possibly it's one of the things that CCC removes), and replacing that made it work again
PS I don't suppose anyone knows any good ways of reducing the amount of stuff on OS X's startup disk? (like useless files or caches that can be deleted, such as the remains of downloaded updates or things like that) Just that with 8 GB it seems a real squeeze, and my startup disk seems to get fuller and fuller even though I'm not installing things to it!