'Best' is subjective, but from the results of GBAtemp's vote, the Acekard 2i is the best flash cart.
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You cannot play DS games with the standard SD card; the SD card is too big. However, if you have a flash card (R4, SuperCard), you can store .nds games in your micro SD card. The flash card will be able to read the .nds game files and you will be able DS games.
Find the packaging for the official website of the flash cart you have. The firmware for it will be on that website.
How about you don't, and just buy games legally, instead? :)- no, I want the HOMEBREW games and apps
pizzatron3000 cart surfer and fishing
There are many different companies the sell rolling utility carts. Quality carts are more expensive and the best cart varies depending on price range.