According to info from Samsung, the Eternity can play the following video formats...
Supported resolutions are 176x144 or 320x240 at 15, 20, 25 or 30 frames per second (fps).
The 320x240 resolution limit is a major buzz-kill for me. My ancient Zodiac PDA (2003) can play full screen 480x320 video at 60 fps, but this much more modern device can only do half that. So if I want full screen on the Eternity I'd have to encode at only 320x192 rez and zoom. That's pretty pathetic, especially considering how nice the Eternity's 400x240 screen is.
For info on converting videos check out videohelp.com, that site has a wealth of information on video formats and editing, authoring & converting. For software that can convert pretty much any video to any other format, check out http://www.erightsoft.com/SUPER.html