With the new flashcarts nowadays, you don't need to only have 1 game on your sd; bigger capacity ones are actually pretty cheap.
If you want to start playing right away, i'd think a N3DS, a Sky3DS, and some MicroSD off Amazon would be a good choice. It's easy to use and isn't blocked on the latest firmware, but you'll be sacrificing the ability to play online (easily at least) and use homebrew. It's also a novel method of switching from game to game, by pressing the button and waiting several seconds. You can look into the Sky3DS card some more
here.
Alternatively, if you want to wait and get a card with more features (homebrew, easier switching, emulators), I would go for a Gateway. However, it would also be more safer to hold off on buying a N3DS until Gateway support comes out for N3DS, as anything could really happen right now. Read
here.
There's also the mysterious DSTwo+, an upcoming(?) cart which might be good enough to compete with the other two. Might.
Best bet IMO is to hold off on getting anything until more info comes out about the Gateway/Sky.