I posted about this before, but here's a quick recap:
SD spec is up to 2GB - the new[er] SDHC cards that allow 4GB and bigger are NOT compatible with "SD Only" devices like the Wii (older digital cameras, etc.).
A few manufacturers managed to make 4GB cards on the SD platform - however the SecureDigital Association has stated the only 4GB and greater cards within spec are those conforming to the SDHC architecture.
These are a *huge* crapshoot since they're "out of spec" - there are issues with the memory buffers, address lines, FAT v. FAT/32, etc., etc.
Bottom line: I wouldn't risk the integrity of my data with a card bigger than 2GB on SD spec devices.