Thanks for all!
@callanish : I'm going to try the allok video to ipod converter soon!
@pycage : It doesn't matter with ipod earphones, since I still can listen music well with them, just when it comes to movies, then there are problems.

You guys know that the resolution of N8xx is 800x480. So my main purpose is that playing the original *.avi file without any converting with Mplayer, and it has been working well (except the sound is very soft, and when using SD card class 2 or 4, there may be some lags). Finally I guess I gotta convert to boost the volume.

Anyway, still welcome any recommendation!