You can't change the core PowerPC platform. You can max out the memory, put a newer possibly faster hard drive in (if it hasn't already been replaced recently), maybe an SSD, and that's about it for hardware.
For software, the last operating system you can use is OS X 10.5 Leopard, with 10.5.8 required to sync with iPhones. Apple no longer sells 10.5, so you need to buy it from a merchant or used. One source of new unopened Apple software is http://www.hardcoremac.com . If you go ebay or craiglist or the like, you'll want a black-faced 10.5 Leopard disk, and not a gray-faced disk, as there were none delivered with Powerbooks, and the gray-faced disks are locked to the system type they are delivered with.
Even with that, there will be limitations. Flash, like it or not, is no longer being developed for the PowerPC platform, and the PowerBooks have difficulty keeping up with streaming audio/visual today. Some web sites even require Intel processors to see content.
The PowerPC processor has pretty much fallen off the radar screen. It's getting hard to find new software that stil runs on a G-series Mac.
That said, you can upgrade your PB to Mac OS 10.5.8 if you can find it. Apple no longer sells it and legit copies on the open market are getting pricy. iDevices will sync under 10.5.8.
In spite of what the outdated Apple Specs say, yo ucan, with moern RAM modules, upgrade to a total of 2GB RAM:
Mac OS X 10.5 Leopard installation system requirements
Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).
If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:
MB427Z/A Leopard 10.5.1 install DVD
MB576Z/A Leopard 10.5.4 install DVD
MB021Z/A Leopard 10.5.6 install DVD (single user)
MB022Z/A Leopard 10.5.6 install DVD (5-user family pack)
Installing Mac OS X 10.5 Leopard
Mac OS X 10.5 Leopard Installation and Setup Guide
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
The DVD should look like this
Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.