Having just beat Gears of War(a great game), I have to say i didn't see a huge leap in graphics. From what it looked like it had about the same graphics(on a regular tv), except more detailed characters, textures, and better lighting than certain games iv played on gamecube...but not by much.
the first thing you notice is the characters are really detailed, they have all this armor, and their shiny. Comparing it to Resident evil 4, you could say these charadcters look more detailed, however having played twilight princess I have seen characters just as detailed.
the backround is vast though and the draw distance is pretty high, but then again so was twilight princess. When you ghet into the game you start going inside more and then it ends up looking just like resident evil 4. Even some areas like the caverns and buildings look like they were taken out of re4.
the enemies are detailed, but usually blown away before you get a chance to see you. They are comparable to your character in appearance, but than again there are never that many people on the screen at a time.
whats really impressive are the huge monsters you fight, but then again if you remember star fox adventures you woul remember some pretty big and impressive characters, as well as some on resident evil 4.
The lighting is better, but than again iv seen what Super Mario Galaxy looks like. Also, eternal darkness probalbly had better lighting than alot of games out now.
What really i think is what makes games look morte next gen is the textures. One of the problems with the last generation was that the characters were looking really detailed, but the textures in games were still blurry. Gamecube had amazing texturing abilities(better than xbox, but what happened was a lack of memory)
from reports Wii has more power, but not much. However it has alot more memory, and also alot more disk space. Wii uses DVD 9 disks which are 8.5gb, Gamecube had a measly 1.5 gb, gamecube had 48mb, and apparently wii has an extra 64mb. Chances are the original gamecube was plenty powerful, but there comes a limit what can be produced graphically when there is a lack of memory. That problem is fixed with wii.