Ive looked for this answer, but to no avail.
1. what format of video does mythtv support

Ive got all my dvd's on a hardrive, Just trying to get mythtv to read them.

2. do I need to convert them to a different file format? if so, which?

should I be looking for codecs for mplayer?

The default player used in MythVideo is mplayer which plays a ton of formats and codecs are available for a ton more. The most common format it plays out of the box are mpeg2, mpeg4, xvid, divx, and VOB.
I highly suggest Graysky's guide for updating Mplayer, codecs, and XviD after install.

http://mysettopbox.tv/phpBB2/viewtopic.php?t=4706

What format are you looking to play?

got what I needed to work in xine, but video is a little jumpy
its dropping too many frames. would i586 have anything to do with this and what is the difference between i586 and i686?

That would depend on the processor that you use as these are to suit different architecture. You could also check TOP while play your video to see if your process is able to keep up.

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Girkers

Also try turning deinterlacing on (within Xine setup). I do the same thing and my playback is as smooth as when using a DVD player...

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nbdwt73