Ok, so the two on top are for Gamecube Memory cards (only). The SD card slot CAN hold info, but the system doesn't run on it automatically.
The system itself is mostly the storage you'll need. The SD card slot is for stuff like Pictures and mp3 files. Handy when you play Excite Truck (which can play your mp3 tracks mid-game).
Other than that, I don't think there's much use for them (oh, except a couple of features from Brawl (the most anticipated game of 2008), which include cutomising levels and recording videos).
Oh, and the largest you can use is 2GB I think.