I think its on topic to include the PC in this discussion, otherwise PC/Wii owners will feel left out.
I think the Wii definately has an edge over the PC in resolutions like 480i/480p but it quickly loses it as the resolutions climb whilst the PC is the oposite, no game is playable at 480i that looks as good as a Wii game yet when the resolutions increase it shows some incredible legs all the way out to 2560/1600.
Theres some debate about which one is better, fanboys on the internet often cite games such as SMG or Crysis, depending on which side of the fence they are on. I hope Happy Squirel doesn't chew my butt out for this, but I perfer higher resolution gaming and I like the fidelity of Crysis better even though I don't have any evidence to back me up as far as which ones better.
The PC should be able to produce better graphics at low resolutions to the Wii, but being that most PC games are designed around resolutions of 1024x728 (as a minimum) at this point in time the games look awful at lower resolutions. In contrast, Wii games are designed around the limitations of lower resolutions, and at higher resolutions the pixelation of textures and polygonalization artifacts become very noticeable and thus would not look particularly good.