20gb is nothing, those things are almost obsolete and they haven't even been released yet.
When you start talking about 300gb+ on a disk, and upto 1.6TB, then you can colour me impressed:
http://en.wikipedia.org/wiki/Holographic_Versatile_Disc
http://en.wikipedia.org/wiki/Holographic_memory
http://www.theregister.co.uk/2005/11..._holo_storage/