Video is built in unless it has already had a third party PCI card installed. Some models of that machine had an enhanced A/V IO card to run video from a camcorder for example into it or S-Video out to something, like a TV.
All the other parts can be replaced, but except for HD and CD only with the same kind.
The CPU is upgradeable. It is called an Encore ZIF card. There were 300 and 400 Mhz G4 cards too besides what's in the link below.