Josh. said:
Try formatting the SD card by computer. Just right click the icon and click Format.
this.
insert the sd card into the slot on your computer, or put it into a camera and connect that by usb.
go to "my computer" and check for KODAK, after J:/ or some other letter, in removable disks.
if it's blank/nothing's on it, go back to my computer (remembering which letter is your SD card) and right click the sd card, then select format.
on the options window that will come up, click "Quick Format" and then hit ok.
try that, and see if it works. if not, try a SanDisk. you won't need anything bigger than 1GB or 2GB, as you can always erase the pictures when the card gets full, and just keep them on your computer.
you can also save Virtual Console games or backup your Wii Game Saves (some of them) onto your sd card, if you are running out of space.