5kbytes/sec is pretty safe, 6 is probably pushing it once you get add in overhead and other losses.

Another thing you can do is create a reference movie that can look at the speed the user has specified they have in QT's prefs and automatically choose a version of your video. So you link to one file and a 56k user might get a 5k/s version, a mid DSL user 30k/s, and a fat bastard pipe user 150k/s.