Hun,
I will be testing the Viewsonic 47" 1080p LCD (LVM-47w1) next month so I'll try and give you an idea of the major differences. From what I've seen and read, there is a noticable difference in fine detail with top quality HD sources when comparing 1080p to 720p/768p displays. Most viewers do notice the difference while some claim they do not. I have also read that non-HD sources look worse on 1080p displays as they are more revealing.
As for why there are so many 768p displays as opposed to all of them being 720p, it is because LCD panels have their roots in computer resolutions, and the stardard resolution for many years was 1024x768 (XGA). When the first Wide panel XGA (WXGA) were produced, they increased it horizontally to 1280x768. This worked well for PC Monitors, but the 15:9 aspect ratio does not work for TVs. The panel makers decided to stretch it wider rather then cut off some vertical to get to 16:9 ratio, and that's how they got to 1366x768.
Finally, all sources are scaled to full screen with 768p displays, so whether its 480p, 720p, or 1080i, they will be scaled to 768p with no borders.