D.P. said:
They mentioned in parsing that the main processor was around 1.5times as powerful, being Nintendo this will be a conservative figure.
But somethings are suspected of being more like twice the power, such as the amount of memory, which allows bigger worlds, higher res textures, more details, less loading times, etc. The game disc has a bigger capacity allow more content and higher res movies for cut scenes and better online media streaming. The graphics processor is rumored around 1.5-2.0 faster clock speed, but with internal improvements and optimizations, better memory controller etc. And there is better drivers and firmware.
Plus being non-HD means an 1.5x is actually 1.5 times. The jump to HD resolutions requires 4 times the memory just for the frame buffer etc. and requires 4x the memory bandwidth to shift the data.
So the wii should provide nicer graphics than the GC, we just haven't seen this yet. games like MP3 and Mario Galaxy should show what it is capable of.