My personal preference would be to use Flash video (FLV) at 320x240 pixels (assuming 4:3 aspect ratio).
You can generally get pretty good results at 15 fps using the high quality setting.
With good editing and voiceover you can pack a lot of information into a one minute video clip.
The resulting file size would be a fairly bandwidth-friendly 4 Mb or so.
Thanks for the comments. Pretty much what I was hoping to hear, especially the length. I was actually thinking in terms of some very serious editing and 30 seconds unless it was a really interesting street scene or something like that.
I will have to learn how to do quite a few things, especially Flash video .. and that scares me a bit.