Wii games will be playable on Wii U, unlike GameCube games. They won't look any better than you remember, however, as Nintendo has announced that Wii U will not upscale Wii titles.
Nintendo of America Reggie Fils-Aime revealed the news in an interview with GameTrailers this week. "It will not do that," he said when asked about the possibility. "That's really driven by the hardware, and the original Wii games were built for a specific type of hardware. There's no mechanism to quote-unquote 'up-res' that unless we launch a brand-new game." Giant Bomb also confirmed that this is the case with Nintendo PR director Mark Franklin.
It's disappointing news, especially in light of what we've seen from a PC emulator that played Wii games like Super Mario Galaxy (the upscaled version of which is pictured above), Zelda: Twilight Princess, and Metroid Prime 3 in 720p.