You can't change the processor in your Powerbook. You didn't indicate which model you have, however, most aluminum PowerBooks are able to run 10.5 Leopard, which is the last PowerPC OS X release. Leopard will also run the current iTunes (although HD video won't work, as that requires an Intel processor). Leopard requires:
- A Mac computer with a Power PC G4 (867 MHz or faster), G5, or Intel processor
- 512 MB memory or more (1GB is better)
- A DVD drive for installation
- 9 GB of available disk space or more
PowerBooks still have performance in them. Where they are failing is with streaming internet video. Unfortunately, the world has left the PowerPC platform behind. There are some things you can do, for example, you can view YouTube video's in HTML5, which helps keep audio and video in sync. You can't, however, watch anything on ESPN as it requires current Flash, which in turn requires an Intel processor.