YouTube – How to Find the Licensing Agreement of a Video

Firefox add-on SmartVideo for YouTube will do just that. You can even tell it to either start the video when you want. You can just open the link in a new tab, and just come back to it when you're ready to watch the vid. It has other useful options as well.

Edit: SmartVideo can be a bit buggy. As to 720p videos being buffered, I think it's because all videos in 720p (or higher? not sure) are already on WebM format. It's my default setting. It's the fastest video setting to load, faster than a 480p flv file.

Edit2: SmartVideo is also available for Google Chrome: https://chrome.google.com/webstore/detail/smartvideo-for-youtube/lnkdbjbjpnpjeciipoaflmpcddinpjjp?hl=en

According to the FAQ, when YouTube detects an infringing video there are three options for the copyright holder:

There are three usage policies—Block, Track or Monetise. If a rights owner specifies a Block policy, the video will not be viewable. If the rights owner specifies a Track policy, the video will continue to be made available on YouTube and the rights owner will receive information about the video, such as how many views it receives. For a Monetise policy, the video will continue to be available on YouTube and ads will appear in conjunction with the video.

If the copyright holder has chosen 'Block', then your only option is to contact whoever made the music (or the label that owns it) and ask them to use it. Maybe Wikipedia can help you find who that is.

Gangnam may have the "monetize" option selected, which would make sense since YouTube has estimated to have paid $1.7 million in revenue for it so far.