We've posted dozens of high definition/quality videos to YouTube http://www.youtube.com/view_play_list?p=C840BBCB48E7F225 , and while the audio remains good, there's always a reduction in resolution.
However, depending on the recording software you're using (we use Camtasia) , you can improve the viewability of a video by zooming in, or using frames, arrows, etc., to call attention to certain parts of an application or the desktop.
