Well, I tried some of the mplayer command lines by myself. It seems that the best performance I can get on my SL7500C is with:
mplayer -nosound -fs -vo w100
First, I can't get acceptable performance with sound enabled even after trying all possible "-ao" options. Does anyone know how to get sound working smoothly with video?
Second, -fs, I found that with "-vo w100", there is no speed penalty scaling to full screen.
Third, among all the -vo options I tired, "w100" is the only one that provides smooth video. Other ones (even /dev/fb0) have screen flickering issues.
Also, I figured out that IMAGEON 100 has only 384K video memory, But I'm not sure why 512x384 is the max accelerated size. (iczer3, can you explain this in details). I tried 320x240 and 640x352 video sizes and apparently the latter one was not accelerated and end up with a lot of flickering.
I'm still expecting beta4 for C7x0 to try out some new options.