Really, no one uses all the space on a DVD. (Few exceptions.) In fact, I believe that nearly all of the games are on CDs, not DVDs at all. Besides, if they can put Resident Evil 2 on the N64, the space between the discs is minimal.
Also, the idea is not direct ports. If need be, a company could change the format to use two GCN discs (I assume)...
The systems, all three, are really on a level playing field. It\'s going to be interesting to see what dictates market share.
-Eik