After some more tests, i finally obtained this setting as the best :
Encoding with HandBrake (mac os) gives a lot less mosaic,
576x352 (slightly cropped on both sides), MP4, 750 Kbps
MP3 44.1 KHz, 128 Kbps

I then get very nice fullscreen and detailed playback.

Qole, thankyou for suggesting 576 instead of 640 ... it was for testing, but finally, it seems to work better than 640 ...

Daperl, what you suggest is very interesting, i suppose a real movie, and even more a "dirty" one would give more compression artefacts, not necessarily a less good performance, what do you think ?

I will try to make another test with a real movie.