I'd say most games, but some are 4:3/480i only--one big one that is is Mario Party 8.
Also, a few have underscan meaning it's widescreen but you'll have small black bars on all 4 sides. Metroid Prime 3 has this, among others.
The games that are 16x9 and 480p look pretty good, but they are a bit jaggy and low res. Don't go in expecting it too look anywhere near as good as the 360 or PS3. Metroid Prime 3 (tiny black bars aside) looks pretty good IMO.
Honestly, if graphics are very important to you, the Wii isn't the console for you as it's just a slightly souped up Gamecube in terms of graphical power, and the lack of HD support is killer for videophiles.