Now you can view YouTube videos in HD (High Definition), at least those videos that have been uploaded in that quality. Recently YouTube has adopted a widescreen format for all of its videos too.
While playing a YouTube video, you can see the link “watch in high quality” to the bottom right corner of the widescreen video. currently the HD option only appears on videos on YouTube itself, not in embedded videos.
The High Definition videos take more bandwidth, so use them only if you have a faster connection and bandwidth to spare for videos.