Never heard of the N-Card, so I wouldn't be able to say anything about its quality other than I would be inclined to think it would be a cheap knockoff.
What I can tell you is that you won't be able to easily play gba roms on there. None of the slot1 cards can let you load gba games. Its a limitation of the DS hardware or some such, I think along the lines of how the gba couldn't use GB Color carts or something.
You *can* still play gba roms, but you'll need to use a gba emulator that runs in DS mode. I haven't looked at any in a year or more, but last I saw they were still slow and buggy.
A user review I found says the N-Card supports DLDI autopatching, so assuming that is true, you shouldn't have any issues with DS-native homebrew and would have full read and write support.