Haha, you know a Wii is just a souped up GC right? Mario Galaxy does look, but thats because of the all the effects to hide some of the raw stuff.
How much better can the Wii graphics get? They're reaching the limit soon, ok, my bad about saying the graphics would be shit, thats an overstatement. They will look bad though compared to PS3 and 360. They can make the Wii graphics looks simple and smooth, but you don't get the massive amount of deatol you can get with the other consoles. The number of people on screen etc...
The Wii will stop somewhere graphically, probaly in mid to end 2008, is where the graphics will be the pinnacle.. They will use DoF and bloom and such to hide the nasties..