a great little piece of equipment but not all that easy to update.

Most everything you can do to upgrade it requires major surgery.

The processor can be replaced with faster one, if you can find a manufacturer who still makes them, Sonnet had one if I recall.
PowerLogix also produced one:
Check here for Processors.
If you get a faster processor, get the 80mm fan as well. You will need it.

The graphics card is also going to be difficult. Any replacement will have to have Mac compatibility and all of them will need to be "modified" to fit in the available space. Usually involves cutting pieces out of the card.
Check here for more Cube news:

HD and Optical drives can be changed quite easily but that does nothing for speed and video.

Hope this helps a little.

FWIW, the Lady Wife currently uses a G4 Cube as her main, email and some surfing, machine.