Yes, you can upgrade it. However, it will void the warranty of your hard drive, and if you break anything else in the process of installing the hard drive, that won't be under warranty either. You can find how to take apart your Powerbook from the Apple Service Manual, found
here.
Or, you can buy the hard drive and have it installed at an Apple Store or Apple Service Center, for I believe around $30.
And you cannot upgrade video memory, it is mounted directly on the logic board.