Portable Media Player?

Why spend a lot? You already own a GBA, GBA SP, and/or a NDS.

The GBA Movie Player enables you to use your Gameboy Advance™ or Gameboy Advance SP™ to play video and audio files and read e-books from standard Compact Flash™ cards. You can use your PC and a normal Compact Flash card reader to transfer and convert your media files to the Compact Flash cards.

[ lik-sang ]

You'll need some CF cards to port the video to, SanDisk, Lexar, and Kingston all seem to make relatively cheap ones.

There are some issues discussed on Lik-Sang's forums which make this seem like a bit of a pain, but I'm guessing that these represent power users, newbies, idiots, and people helping the aforementioned.