Try youtube HD videos.
the best video will be the same res as the display (1024x768) DVD's are gonna be a little under (though if they're full quality, not iTunes downloads or internet downloads/streaming they should still look good) at 720x480 and 720p is just a bit bigger at 1280x720 (bigger is better than smaller)
anything smaller than 1024x768 isn't gonna look as good stretched to the ipad screen.
If you are looking for Music Videos for the iPad, DaPOW.com is the place to go.
Continuous Music Video Episodes (for all of you 80's Music Television Junkies Out There Needing Your Fix...)
Vevo HD.......Best way to watch videos on iPad