There's no such thing as a good SD card. An SD card is an SD card. The only differences that actually count, are the capacaties, really. A Toshiba 2GB is no different than a SanDisk 2GB. Also I have a 32 GB SDHC card, which is the highest it can support so far. A 2 GB might be enough for a while, but then you'll eventually run out. If you have 32 GB, while you might think it's a lot, possibly too much, you never know. You might end up having like 10000 or more photos, or have downloaded enough software, etc to fill it up. Or, on the other hand, if you have a 2 GB, there's that chance that you'll have eventually filled it up, just as that game you were waiting for is available in the eShop, however, with 32 GB, you can fill 16 times as much space. IIRC every single Game Boy game and Game Boy Color game ever released in the US I think would take up enough space on a 32 GB SD card to still leave... well a few GB at least. I think all GB and GBC titles come to about 10 or more GB? Maybe even less?
Like Link said, if you're not planning on necessarily getting much (maybe getting a few eShop titles, and storing some extra data), a 2 GB or maybe 4 GB is OK. However, if you simply want "virtually unlimited" space, so you can download every title that's available and save over a few hundreds replays, or other extra bits of data you can like replays in Ridge Racer 3D, and presumably ghosts in Mario Kart 3DS, and still have enough room to hold a few albums to listen to with 3DS Sound, and a few thousand photos in 3D, get a 32 GB.
In short, it's up to you. Do you want one that's just enough for you to store what you want, or one that allows you to store as much as you want? Think of it this way... would you rather have enough money for maybe a few games, or enough money to buy the world's most valuable mansion, the most expensive car in the world, and even take over a very very big company, such as Sony, and still have enough money to buy all the contents of Buckingham Palace?
