If the main thing you want to do with your DS is play movies, you might consider getting an M3 instead of a supercard. I do hate saying that, but currently it's built in media player is still a bit better than moonshell at playing movies (Moonshell is the homebrew freeware program you need to watch movies on supercard). If you do go the supercard route, I have heard that the CF version is slightly better at movies right now than the SD version. Though the SD cards access faster, the driver moonshell uses to access the SD card is slower than the CF version. This is something that will probably be fixed soon though. Either way, be sure to get huge media as movies take up a lot of space. You should get 2gig media, though the supercard will take 4gig, its brand picky and the format procedure is a bit complex.
Either way, definatly check out moonshell as its one of the best portable media players out there, and does a pretty decent job with video. The full download will include the video converter and enough documentation.
I have heard the following settings are ideal for the convertor. It's about 60MB for 30 minutes:
PCM Rate: 32000
Volume: 160% (the DS is very quiet normally I listen to MP3's at 200+%)
Check Enabled re-encode video stream.
Screen width size: 256
Aspect ratio: auto detect
Bitrate: 256
Frames: 12
Brightness: 110%
blur deep: 95% light
I know most of what ive said is "I heard this or that", but I don't really watch movies on my DS as 1 movie=12GBA games=8DS games=80MP3's=All decent (U) NES games ever, and I would rather have all of the latter 4 on there.