Does it upscale GameCube games? If not do the still look ok? Dreamcast VGA 480p still looks great.
If you want the clearest 480p, the original Wii will do. With component cables and a compatible TV, it should be just as clear as DreamCast VGA.
For many games there are even community high-def texture packs that you can add to make them look even better.
[Edit: Sorry, was talking about Dolphin here, not Wii U!]
Wii U's max HDMI output is 1080p. Also I have not seen any information that it does upscaling at all, so if you have a source for that, I'd like to know it.
There are a handful of GameCube titles that were 16:9 native widescreen on the original console. The one that comes to mind for me is Serious Sam: The Next Encounter