It does have better than the gamecube, just look at some of the shading in hi-res videos of Mario Galaxy to see that its more capable than its counterpart. I mocked up this quick side by side (better ones exist). Both are at 640x480 resolution to make a direct comparison.
Mario Galaxy vs Mario Sunshine

It's more powerful than the gamecube, approximately 4 of them by earlier estimates. It's no Xbox360 or PS3, but its easy to program for, cheap for all sides (manufacturing, game production, and consumer), and you don't need to buy a new TV to see the real benefit.

Just think how much the graphics improve over time on all the previous gen systems (especially the original PS). The architecture for the Wii is basically the same to the Cube. Instead of floundering about with new hardware, programmers already know how to push the Cube to its limits, and now they've got four of them in one box. They will easily be able to pick it up and max it out.