Honestly, you can have some very impressive games on the Wii depending on how willing (and able) developers are to "Cheat" (by cheat I mean pre-render) ...
I'll teach you a trick that is possible that would mean that even if the Wii was simply as powerful as the Gamecube the extra memory in the Wii would enable certain games to look far better than anything the XBox or Gamecube ever had. Many (if not most) games have very static environments with static lighting which enables lighting to be precalculated. You can (essentially) generate a Z-Buffer for each light ahead of time (so that you can correctly apply shadows from dynamic objects onto the environment) and generate a texture on the object (for each light, and one for no light) which stores "how" the light impacted that object; you'll have a limited number (probably 2 to 4) of lights that you will want to allow to have an impact on an object but this should also allow you to use more advanced lighting techniques (and emulate higher polygonal detail) than the Wii is capable of.
Edit: I should mention the Z-Buffer technique I mentioned is a form of shadow mapping which has been used on both the XBox and Gamecube. It is used (mainly) because it allows for self shadowing which is difficult to do otherwise.
