basically do you want NDS or 3DS roms, if your after 3ds the only cards that will work are gateway MT or one of the cheapo crap clones, but just so you know, they will only work on a 4.x system
if you just want NDS roms, one of the box standard r4 gold card will work, but make sure its one of the newer revisions like this
http://www.dsgamerworld.com/r4i-gold-3ds-rts-flashcard-p-2.html
NDS flashcard are capable of running all the way upto the latest FW released 9.x as long as you dont buy a really onld card that has been blocked,
if you want the best possible NDS flashcard you can get the
DSTWO, but it is quite a bit more expensive, but it has more advanced features and is able to emulates snes etc as it has a processor built in that makes it the most powerful card on the market