Nothing on screen during power up

The iBook G4 has enough RAM on the motherboard to function, so to troubleshoot whether or not your extended memory is the problem, just remove it and see if the behavior changes.

Is the screen black, or white? If it's white, and assuming the hard drive is blank, you're probably not seeing anything on the screen because the laptop is looking for something to boot from and not finding it. Try holding down the option key as you power on, and this should bring up the option menu, which lets you choose from the available (if any) bootable devices. If this menu appears, that is a good sign, and hopefully you will be able to insert the OS media, choose it in the option menu or by booting holding down "c", and then reinstall the OS.

If the screen is black, try removing extended memory as I mentioned above. Besides that, resetting the PMU and PRAM can never hurt and can sometimes jolt a computer out of a non-functional condition.