Japanese Kingston is probably the most recommended microSD card.
Format with FAT16, but due to a limitation on the number of files that can be on the root of a FAT16 file system, be sure to put your .nds files and saves into a folder (not on the root of the microSD card).
Also note that a microSDHC will work with a slot2 GBA expansion pack (such as the EZ 3in1 or eWin 2in1), but not with a standalone slot2 cart. You only need a GBA expansion cart for use with a DSTT to play GBA games and to act as extra memory needed for the Opera browser and some homebrew. With a GBA expansion cart, your games are stored on your DSTT's microSD/HC card, and when you want to play a GBA game it is transferred to the GBA expansion cart's internal memory (these generally have enough to hold a single game in memory) to play the game. With the DSTT you'll need to use the GBA ExpLoader program (available in the download section) to load the game to the slot2 GBA expansion cart.
For larger-capacity microSDHC cards, it is recommended that you get a class 6 (A-Data is a popular brand). A microSDHC will need to be formatted using FAT32. You should still rput your .nds files and saves into a folder.
-Bri
