download Youtube downloader at http://download.cnet.com/YouTube-Downloader/3000-2071_4-10647340.html
(this software is safe to use. I use it for years). This will allow you to download any videoes from youtube.
After download, you need to convert the .FLV file. When you do that you have to do it in "Manual conversion". (you need to change it on the bottom) Click "OK" will bring you a popup window. On the bottom bar put this code
ffmpeg -i %1 -vcodec mpeg4 -qscale 1 -s 432x240 -r 29 -b 1500 -ar 44100 -ab 128 -ac 2 %1.mp4
and click OK. This will open a new MS-DOS window and show some work. wait until it finish. Now a full screen video is ready for M800. Now all you need to do is upload this file to your phone.
Very easy steps... and fast convert.