Likewise, I don't think you can control QT's display relative to the download status of a clip.
In OS X 10.5.8, running QuickTime 7.6.6, I see the following in the
System Preferences ->
QuickTime pane ->
Streaming tab: an
Enable Instant-On checkbox, followed by this caveat: "Instant-On starts playing streamed media without delay, but network congestion may reduce the quality of playback. Choose a longer delay if you experience problems."
Below that is a slider which allows adjustment of the delay length.
(Note that under the
Browser tab in the same prefpane, there's a
Play movies automatically checkbox. I imagine that's checked by default, but who knows?)
