Q: I know only one size of SD card will fit the slot, but do I have to buy a Wii-specific one or would a standard SD card do the trick? And do I need a gamecube memory card to save gamecube games?
A: SD cards are SD cards, it doens't have to be Nintendo branded. You do have to have Gamecube memory in order to play gamecube games. I don't even think you can copy Gamecube savestates to an SD card for backup like you can with Wii savestates and channels.