When i had the 512mb ds extreme it seemed to only support NDS files. When a game is saved it will create a SAV file for that game and be stored onto the card.
Diveno is a good place to get it as i saw it on there for around 70-80 pounds.
The actual size of the card is 2GB. the 16gb means 16giga BITS which works out to 2giga BYTES. So the card has a 2GB storage because the old one confused people into thinking it was 4GB when it meant 4giga bits which worked out to 512mb :D