1080i is as said, interlaced. The screen is updated twice per refresh, once odd lines, then even lines. Can give a (not very noticable) flickery picture.
1080p is progressive. the whole screen is updated in one go, giving a rock solid picture.
can most people tell the difference - no.
Yes samsung sell panels to other people. There are only 2 or 3 people in the world that manufacture LCD panels. HOWEVER, all LCD panels are not equal, even from the same supplier. They all have different response times, brightness and contrast. To make it even more difficult, each panel is graded as to its overall quality.
Don't think that just because company x and y use panels from the same supplier in their panels that they will be the same though. The picture quality depends wholly on the image engine that drives the picture, for example, bravia and bravia ex to name a couple.
as you have no high-def sources, i'd either get a cheapie 720 and upgrade when you get a HD source, or get a 1080p that does a good job of upscaling non HD content.
As such, you're looking at either the top or bottom of the market.
