Modern plasmas don't suffer from the old dreaded burn in any more.
720"P" and 1080"I" are pretty much the same quality, 720 progressive and 1080 interlaced, and pretty much any tv that's capable of one is capable of the other, your confusing 1080i with 1080p (progressive) which is where the big price difference and improved visuals come into play.

720p = 720 pixels of vertical resolution scanned every hz, 1080i = 540 lines of vertical resolution scanned every hz (it only draws half the image during a scan, then the other half the next scan, making the whole 1080i image over two scans). 1080p = scans 1080 lines of vertical pixels every hz, like the 720p but greater resolution (and much higher price).

For what it's worth pretty much the only thing that puts out and actually uses a full 1080p signal is a bluray player. All of the xbox 360 games only output in true 720p or 1080i, same for the ps3 games, (due to performance restraints). Blurays movies can utilize 1080p but it's not likely you'd see any difference from 1080i unless you ran them side by side. Oh and almost all HD programming from cable companies is also predominately 1080i. In other words, 1080p is cool but not necessary, a good 720p/1080i set is all you really need.

Running a 55" Sony lcd with an Onkyo receiver, 360 and ps3, myself, no complaints.

 

Yeah, and whatever you do, do NOT make your decision based off of which picture is more "vivid" on the showroom floor, the display tv's are all set with the saturation and contrast jacked up to catch your eye on the floor. You don't really get a good idea of what a tv is really capable of till you dial it to "true" using a respectable calibration set, or at least a decent calibration dvd. The default settings used on them is usually called dynamic, vivid or some such nonsense and it washes out any detail the images onscreen might have, go into the menu and switch it to a more normal setting to see which set actually has a good picture.

And whatever else you do, don't take the salesmans "word" for what is good. "My friend/parents/me/whoever just got that set a little while ago, the pictures amazing, they love it" is one of most common things you'll hear, wait ten minutes and you'll hear him say it to someone else about a different tv, heh. No offense to you Mitch, you know what I mean tho, most salesmen don't know squat, there's always exceptions to the rule but most people are better off writing off all clerks advice than trying to weed out the good from the bad.