Of the .avi files I have copied (un-transcoded) to my N800 and viewed with Mplayer, I have had good results. Maybe I have been lucky but .avi (DivX/XviD) files play well so long as I open them with Mplayer only. The other players can't do what Mplayer does on my N800.

I have taken .wmv files and they are poor, but the utility I found called Media_Converter_Windows_1.43 recoded problem files so they play just fine on my N800. I know you said you don't want to transcode (I didn't want to either) but that utility makes it a snap. It has a small foot print, the interface is very intuitive, it is fast, and it remembers the last settings I used and uses them the next time.

I read a tutorial written ( or published ) by Nokia specifically addressing how to optimize video for smooth / clear playback on the Internet Tablets. I'm sure you can find it with Google. Reading that wasn't necessary but it was worthwhile for me.

I obtain .avi files that are randomly obtained from various places, and tossing them on a SD card and in my N800 (without prior optimization) results in good playback (with Mplayer) most of the time. But as I have stated, maybe I've been lucky.

The rest of the N800's functionality more than make up for it's quirks in the video playback dept. in my view. Your mileage may vary. Regards.