256MB is a really decent size. When you consider that most games are between 3-10MB (note I said most), you could install 18 7MB games and only fill up 1/2 the card, leaving the other 1/2 for music. If you want to play emulated games, the roms are tiny, and you'll fit a lot.
For music, you'll want the MP3s at either 96kbps or 64kbps (radio-quality). Anything higher and they take up too much space. Anything lower and they don't sound that great.
Also, a 256mb card will fit a good-quality movie.
-John
