youll need 3 things: an adapter, a pass device, and some media. for adapters, there are plenty of options, but Ill just mention M3, Supercard and G6. M3 and Supercard are both available in Compact FLash and secure digital. M3 is more expensive than supercard, but supercard has less battery life, worse playback for GBA games, and is overall lower quality. G6 has 512MB of flash memory built in, instead of using removable storage. The next thing you will need is a pass device. these are required for starting in DS mode and you can flash the firmware, if you dont want to use one all the time, but you will need one to inittialy flash it. there are plenty available, but all you need to know is that passcard 3 is the one to get, as it is plug and play, and it is the size of a ds card, rather than other adapters that pertrude from the DS and require a game to be pluged in The last thing you will need is media (and a card reader if you dont already have one). there is a test in which the opening sequence of dawn of sorrow is played from various brands of memory cards and the cards are given a reading based on playback quality. youll pretty much be fine with cards that have 4s and 5s. http://www.m3wiki.com/index.php?title=Media_Card
Indeed, i've been thinking of writing a detailed guide like my M3 guide, but i am too busy (and currently too drunk) to do so.
A Gigabit is smaller than a Gigabyte: 4 Gigabits is only 512 Megabytes (0.5 Gigabytes) There are 8 bits in every byte.
can you please use the edit button instead of double posting. anyway, the G6 lite only has 512MB and cannot have more. i suggest that you get an M3 lite instead. and a passcard 3. and a microsd card.
The 2 leading devices now have versions that don't stick out of the DS. Both the M3 and Supercard have 'Lite' versions available, both use microSD cards for storage. So it's up to personal preference wether to get a G6/M3/Supercard.