Hmm kinda looks familiar to this mp3 converter script http://userscripts.org/scripts/show/149416
Using this script with Chromium 12 and Ubuntu 10.04 on a very low-spec hardware (500MHz).
It is very non-intrusive and allows a very fast access to my preffered quality for each video, for watching with VLC (with streaming) or for downloading.
This is productive, efficient and comfortable.
Kudos to the developper for this very KISS script !