Some people are confused about what you need to get the R4 SDHC card working. All you need is the following:

A Nintendo DS, DS Litei

The R4 SDHC Adapter (See box image above). The package should include the R4 SDHC Adapter itself, A Micro SD USB Card Reader, and Plastic case and a languard keychain thingy. (You only need the adapter and the Micro SD Card Reader though
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A Micro SD Card – The R4 SDHC Adapter supports SDHC so you can have anything from 2GB to 16GB memory cards. Most people opt for a 4GB card which can store plenty of files, enough for most people.
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What size micro sd do I need? Knowing what size micro sd card to get for your R4 SDHC DS card can be tricky. Its not that easy to recommend a particular one because it really does depend oin the sizes of the files you intend to use. However, generally, a 2GB card will suffice for light users and 4GB card will do for most other people. If you have a really large collection of music or video files then you can go for the 8GB or even the huge 16GB card.