YouTube has started testing higher quality videos, all videos are not yet available in higher resolutions but should in the upcoming days, to try things out you need to append a special parameter to the existing video URL, appending &fmt=6 to the URL delivers a 448x336 resolution version of the video and appending &fmt=18 delivers a even better 480x360 resolution version, do keep in mind that higher quality videos need a lot of bandwidth and are not yet available for all the videos.