Sorry about my pricing theory. You were right morika, Xbox sells 20 GB hd's for $100...so why should Wii be anymore expensive? Anyway, price aside, it's still a good idea.
In order to make games playable off an SD card, they would have to link the card to the Wii in question...this makes the SD unusable elsewhere. Whereas if the Wii had an external USB hard drive, they could link the Wii to it and no other Wii could use it. If the owner of the drive wanted to sell it or get a new Wii, he or she would have to clear it before it was un-linked from the Wii. Eh?