if you format it, you will lose everything, INCLUDING THE OPERATING SYSTEM. if you don't have the disks, how are you going to re-install?
Oops... Honestly didn't think about that one
Is Disk Utility basically the same as formatting then?
Disk Utility is an utility for drive maintenance, including partitioning and formatting.
The disks that came with your MacBook aren't likely to be useful on your PowerBook. They usually are related to the hardware they came with. When your MacBook has OS X 10.6 on it, it's certainly impossible. The PowerBook has a totally different processor (PowerPC), which isn't supported by 10.6. And even if you did succeed to re-install the PowerBook using the MacBook disks, it isn't exactly legal...
However, there used to be an option to secure empty your trash. I know it was in OS X 10.3 (right click Trash, secure empty...) This might be useful to you, but I have to look that up.
OK, If only deleting "sensitive" information would suffice to you, you could put those files into the trash and, after that, choose "Secure empty trash" from the "Finder" menu. That way your files are deleted pretty safe. If you want to delete certain software, you can delete it the same way in most cases. Just look up the program in the Application folder and put it to the trash. Some (preference) files will still be on the computer, but those usually won't harm your computer. If you're really eager to remove those too, you can find them in the preference folder in your Library folder. They are called something like com.
applicationname.plist. But there's really not many reason to do that.