I don't own a gamecube. Only Xbox. Out of the new systems the PS3 will be the only system that I will not buy. When a system gets to be 600 to 800 US dollars I find them to be pointless. I can build a good computer system for that price so I would just play computer games at that point.
It is weird to see some people get so nervous about the Wii doing well this year. If Nintendo releases 4 million units to North America and they are sold out by Christmas that will be amazing. And it seems from all the preorders and game sites information that's happening. Game companies will not ignore a system that sells 4 million systems at launch.
The only system that really has to worry seems to be the PS3. They might be pricing themselves out of the game.