I am so glad the digital camera (a Kodak one) I decided to buy for my trip to E3 uses SD cards. I had been contemplating one that uses CF cards, since I have another device that uses them, but eventually made, apparently, the right choice.
Then I lost the cable that lets you transfer images from the camera directly to the computer, so instead of buying a replacement, I bought a card reader/writer, which is also apparently a good choice because now I'll be able to back up whatever I save on the card(s) from the Wii.
(It also means I can abuse the Wii's Photo Channel with all that porn I have... >_>)
1 GB is a pretty good standard starting size. I guess 2 GB and 4 GB is good if you can get a good deal on them, why not? I saw an 8 GB one that was ridiculously expensive, but maybe that was a CF card.
The Wii's OS is upgradeable, so unless there's a physical limitation, the Wii can be upgraded to handle larger cards in the future, can't it?
EDIT: BTW I found the article that Rick linked to fairly interesting. If that's really the case then maybe you can hold off on the SD cards, depending on how much space is used for other things, like Wii game saves and whatnot.