I use avidemux to change videos to .avi files that use XVid4 video and
MP2 audio - resampled to 480 pixels wide, for use on my T5. These give
a nice smooth pictures, whereas .ogm files don't play properly, it
seems. Are there any other combinations I should try that might give
smaller file sizes?

Watched "Batman Begins" on the T5 - full length, on battery power (only
used 50%) - DVD quality - amazing!!