The SD mem cards are only necessary for downloading VC games. The mem slots are for GC games. You don't need either if all you do is play Wii games.
But because there aren't too many good Wii games (most can be rented instead of bought), if you don't already own a GC, then it would be very wise to pickup the handful of good-to-great GC games (Resident Evil, Resident Evil 4, Eternal Darkness: Sanity's Requiem, Sphinx and the Cursed Mummy, Zelda: Wind Waker, Star Wars Rogue Leader: Squadron II, Metroid Prime, Metroid Prime 2: Echoes, James Bond: Everything or Nothing, Paper Mario: the Thousand Year Door, Ikaruga, Prince of Persia (1, 2, 3)) That should tide you over until the next must-have game. The VC controller is $20.
List of GC games: http://www.gamespot.com/games.html?type=top_rated&platform=11&mode=all&sort=score&dlx_type=all&date_filter=all&sortdir=asc