Some real good tips for embedding higher quality YouTube videos on a web site can be found at this link:
http://howto.wired.com/wiki/Watch_Higher_Quality_YouTube_Videos
There are several methods, but this one seems to be good for quickly modifying older posts
Add &fmt=18 to the URL of the video and it will look for a higher resolution version, if it is available. I haven’t tried this yet, so I can’t vouch for how well it works.
