PowerBook g4

There is no such thing as an Intel based hard drive: the issue will be physical properties of the drive, not whether the Mac is Intel or G4.

This page has detailed instructions for replacing the hard drive:

https://www.ifixit.com/Guide/PowerBook+G4+Aluminum+15-Inch+1-1.5+GHz+Hard+Drive+ Replacement/227

As you will see it's a pretty tricky operation (and read the comments, which disagree on some points). There is also a video here:

MAC G4 PowerBook Hard Drive removal instructional video ...

This instruction document comes from a manufacturer of suitable drives:

Internal Hard Drive Upgrade for PowerBook G4 Installation ...

I have not attempted any of this so all this is for information, not a recommendation you should do it. Proceed at your own discretion... or you may be able to find a repair show which can do it.

You might want to consider that there is a limit to how much money it's worth throwing at a computer that old (particularly considering that what you can do with it is quite limited), and whether you might do better to consider getting a new one.

That's for you to decide - it's quite difficult, but possible if you feel up to it. It's not up to me to tell you whether you should or should't.

Yes, it's possible. The model released in November 2002 can be upgraded to Leopard. The earlier models can be upgraded to Tiger. However neither of these is available from Apple so you might have difficulty finding install disks. If you still have the disks which came with the Mac you would of course be able to install from that.