Ok I'm sick of people saying the wii is not a next gen console. What are they basing this assumption on? "Its not got HD graphics"? WTF does that have anything to do with being the "next generation" of video game consoles.
Let me explain.
"Next Gen" means that it is the next generation of a video game console. i.e. If your dad has a kid - YOU are the next generation of your family. You are part of a generation - a group of people your age. Thats a generation.
When nintendo gamecube has a kid (the wii) It is the next generation of the nintendo family. It is part of a new generation of games consoles. So it is therefore nextgen.
To be honest this is the most basic way of defining next-gen. You could go on and say its to do with innovation and other such stuff. But next gen does not mean "better graphics".