I just bought a second of these players
black sword
I think that if a player converts to MTV or AMV, they seem to have lesser movie abilities. (Someone correct me if I am totally out to lunch here) The one I linked to takes AVI - the sample files supplied with them were 224x176 Xvid video with Mp2 audio at 20 frames per second.
I like that iMirror linked in one of the above posts but I already had a good experience with the above seller. Plus a 320x240 @ 30fps movie has to be 2x+ the size of a 224x176 movie and these things have very little space. I didn't think that I'd benefit enough from the iMirror's better screen so I stuck to what I knew.