Multiple versions

Allard spoke up and confirmed that there will indeed be multiple versions of the xbox 360 heading out to the market. He told Developers not to bank on there always being a hard drive there. With that said- we can only assume that the hard drive is just an added Multimedia addition.
My question is- how will the system support downloadable content?
There is also rumor of the system later down the line shipping out with a HD-DVD drive.
So far though, the specifications for the upcoming luanch have not changed- so lets expect the 20 gig.