If you were using strictly just the Gateway to run games, the games can vary from 500mb -> 3GB for the actual game. The 3DS SD card would be used for game updates, so it shouldn't be too significant.
The main problem is that when you make an emuNAND, it is stored as a hidden partition on the 3DS SD card. That alone takes up 2GB.
I'd recommend at least having a 4GB+ card for the 3DS as I don't even know if it can make the emuNAND with only a 2GB SD Card
Well, I currently only have a 2GB card for my 3DS (32GB in the red card). So I'll have to order get at least a 4GB card then?