if still relevant, then on my 5230 video was normally played by the built-in Real Player. Just permission when downloading from YouTube, you need to choose 320x240, and the extension is 3gp. Any other options do not go, I tried everything. For the 5800 screen, this resolution is quite enough, and the processor for large files is rather weak.
If mp.4 is needed, then I pinch the video in the Any Video Converter program using these settings:
The video was normally read by Real Player, the sound did not lag behind. True, I must make a reservation that I did not encode the films, only video clips.