Youtube has been trying for a bit not to suck so badly in terms of resolution.
This isn't official and may change or die, but if you add &fmt=18 (case sensitive) to the end of video, you will likely get a mpeg4 version of slightly higher resolution than the flash crap they've been dishing out.
The youtube button currently strips out everything after the ampersand, so you have to link it as a URL.
It's an appreciable difference...
Før:
http://www.youtube.com/watch?v=kdGbJWSxYfg
Nu:
http://www.youtube.com/watch?v=kdGbJWSxYfg&fmt=18
>> Watch this video in lower quality for faster playback