You can get NES games for the Wii via the Virtual Console releases in the online store. Not a huge selection yet, I don't think, but they have classics like Milo's Castle and Mario Bros. You can't store them on a SD card, but if you need to delete one to make room for another on your Wii's internal memory, then you can always re-download the games that you've purchased. However, you'll need an internet connection hooked up to your Wii.

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Anything good in life is either illegal, immoral, or fattening. - Murphy's Law