This is kind of a long process...but it works great! Download "Free Youtube Downloader" from here:
http://www.dvdvideosoft.com/products/dvd/Free-YouTube-Download.htmYou can use this program to download youtube videos to your computer. Download them in MP4 format.
Then download MediaCoder from here:
http://mediacoder.sourceforge.net/index.htmUse media coder to transcode the videos you downloaded into a format that the phone can support. I found the instructions on here somewhere and I'm sorry to not give props where deserved but I forgot where I found it.
I use these settings:
Very High Quality
MP4 Format
320x240 Resolution
25 fps
on the audio tab I gain 6 db
under transcode select "manual croppping" then under mode select "auto detect".
Then all you have to do is import the videos to your phone. You will still have black bars on the sides because the video portion of the behold only supports 320x240 but the image will take up the entire space vertically and the quality is pretty awesome.
As far as watching them from Youtube directly, you are stuck with what you get. Youtube allows uploading of several different aspect ratios and adapts each to fit the same size screen by adding a black background.
I have a 2 Gig card and about 15 vids on my Behold with about 80% still free.
I hope this helps somewhat.