Here are some methods of getting videos on your Xperia X10
If you just want to get a video from youtube then go to savevid and download the video as MP4(Zune, PSP Quality). Then put it in your video folder on your SD card. When you play the video double tap the screen and it will enlarge the video. If you download videos that are wide screen this method works perfectly and will fill up the X10's screen during playback.
To get high quality YouTube or general videos on your Xperia x10.
Save your video url and paste it in savevid
Then download the highest quality flv or mpeg4, I usually don't go above 720p
Then get Total Video Converter
Install Total Video Converter
Drag the video file and choose mp4(mpeg4 mp4) format as the output format
Go to settings
custom-854*480 (the original x10) screen resolution
Put the aspect as normal
bitrate 128 or for music videos
you can put 192
Frame rate custom 30
Video bit rate 1500.
Then choose output folder and convert
make a new preset called xperia x10
width 640 height 480
video codec mpeg-4
frame rate 29.97
average bit rate 1500
save your preset
then choose output folder and file name and convert