Wise man once say, it is not the capacity of the media but how effectively it is used that is important in contructing solid entertainment. Contrary to the previous post, there are a bunch of great games on cd, such as GTA3, Resident Evil Code Veronica X, Devil May Cry, No One Lives Forever, and Gran Turismo 3 Concept.
Not all PS2 games are DVD based, surprisingly, some games can be crunched in at less than 500 MB to be stored on a PSX-format CD-ROM. Oh yeah, and PS2's use the DVD-5 format (allows 4.7 GB of storage), and Xbox uses DVD-9 (roughly twice the amount of a DVD-5).
Cool, how can you tell whether a ps2 game's on a dvd or a cd? I presume then, according to what you just said, that ps2 demo-disks will probably be on a CD as I can't see them taking up more than 500mb, of course it would depend on the selection of demos.