I have a 16GB card and have downloaded NSMB2 and Professor Layton and several eShop titles(Pushmo, Mighty Switch Force, Ketzal's Corridors, Pokedex 3D, Pokedex 3D Pro, Mutant Mudds + Granny Levels, VVVVVV, Dillon's Rilling Western, Bomb Monkey, The Denpa Men, Colors 3D art application + saved artwork playbacks, Cave Story+), DSiware- Shantae Risky's Revenge, Art Style: Box Life, Art Academy, Kirby's Dream Land VC, Link's Awakening DX, Mario Land 2 VC, Wario Land VC, Zelda Four Swords Anniversry Edition, Donkey Kong Original Edition, all 20 NES/GBA Ambassador titles, all the Preview Videos for upcoming 3DS titles (SM3DL, Animal Crossing, Luigi's Mansion Dark Moon, etc) and like 10 demos and still have PLENTY of room.
And yeah, Dhsu, as long as you're using the 3DS you bought the game on, or you've transferred the data from that system to another, you can re-download stuff you've purchased if you delete the orginal download.