You are obviously a newcomer to these technologies, so I suggest searching Google for detailed explanations, or check wikipedia, for the difference between Interlaced and Progressive Scan.
Alls you need to know is 1080p is better, and alot more expensive. Whether the difference is enough to warrant the price remains to be seen. If you don't know alot about HDTVs and their resolutions, I would make sure you know alot more before making your purchase. Depending on the sources you have, like a PS3 or Sky HD or even an Xbox 360, you need to make the decision from that. Because most HDTVs do 720p and thats the native resolutions for Sky HD, PS3 and Xbox. 1080p is something new, and my personal opinion is the ammount of difference is minute, and at 2 meter distance is unnoticeable.
Well 1080p is 1080 pixels wide, so obviously 768 is can't do 1:1 pixel mapping. there are 2 things you need to understand. Via HDMI and Component, TVs can only do 480i,480p,720p and 1080i. Newer models can do 1080p if they support it natively.
Via VGA, your tv is 1366x768. That means it has that native resolution. When displaying 1080i/p it downscales it, it shrinks the input to fit the screen. When displaying 720p it upscales it.
It all depends on the input device you want to use it for.