Nice guide. Broken link on the Supercard compatibility list, update it pls.
You cannot put the supercard SD into the GBA and play videos or music for obvious technical reasons.
The GBA has an ARM7, which works at 33Mhz. The NDS has an ARM7 and ARM9, which together have 99Mhz. The NDS cannot play all popular music formats (can play MP3s well, though) and the NDS can only play one type of video format, because it takes advantage of all hardware acclerations the DS can give. (anyways, the DS can only give at max 10fps for playing back video)