I can't verify this yet, but this is likely to be ENTIRELY incorrect.
First of all, the new iPad can PLAY 720p videos at the full resolution -- 1280x720 instead of downsizing it to 1024x768 as it did before, so there's that right off the bat.
Secondly, the new display is, according to every review I've read, much more than just 4x resolution improvement. And more than just a bit more color saturation, so the reviewers say.
So yes, I think you should expect a noticeable improvement, and even more so if you upgrade those videos to 1080p -- which you can do for free if they were purchased through iTunes.