You've got 3 cards afaik.
A... like 90 slots one.. don't buy this one. A few games, and it's full.
A 251 slot one.. this one is nice if you've got some games. If you have got lots and lots of games you want to play (about 30+ I think, not sure), go for the big one.
The big one got like 999+ slots.. Too much if you ask me, I should only buy this one if you have got so many games that you think there wouldn't be enough space on the 251 one.
MK7!! 3DS friend code: 0688 - 5579 - 0994
Also playing DotA2 beta!