Here is the trick to view high quality H264 with AAC audio

If your Internet connection speed is good and you want to view the youtube video in high quality you just need to append &fmt=6 or &fmt=18 to the youtube video URL

example: original url: http://www.youtube.com/watch?v=uUH-Qwg66bc

448 x336 pixels: http://www.youtube.com/watch?v=uUH-Qwg66bc&fmt=6

480×360 pixels: http://www.youtube.com/watch?v=uUH-Qwg66bc&fmt=18 (MP4, H264 with AAC audio)

Currently this trick works only for small number of videos only.