You should either get a supercard DSTWO or a r4i gold 3ds rts (r4ids.cn). The card TheSockNaster linked is shit. The DSTWO has more features (play .avi up to 640x480p, real time game guide, game speed changer and a gba/snes emulator) but the r4i gold 3ds is sufficient (plays .dpg videos, real time save). Both have great game compatibility and are compatible to the latest firmware without having to be updated.
The card TheSockNaster linked has RTS as well but it's much buggier than the r4i gold's RTS, the menu is slow as hell etc.