The digital beast will continue growing along with HDDs.
That said, 16-32 gb carts will be more than sufficient for the HH. For home console, the same carts + additional DL for superior/1080p assets. Hopefully Nintendo gets rid of the optical drive, they don't even use bluray move playback. Plus, they love their small consoles and having no disc really brings that down (may give back room to allow for a more powerful system?).