Just some corrections..
The General said:
1080P > 1080i > 720P > 720i > 480P > 480i, and 480i is NTSC, I think PAL is like 536i or some crap like that.
1080P > 720P > 1080i - in terms of visual quality
The fact that 1080i is interlaced, degrades the "quality" of the image significantly when you view the output... On a given "time slice" how much information is conveyed?
720p = 1280 x 720 = 921600
1080i = 1920 * 1080 / 2 = 1036800
Interestingly, the # of pixels for 1080i is slightly higher.. But, the deinterlacing operation along with other factors can severely reduce the quality of the output.
The General said:
A 720P HDTV is generally 1366x768, whereas a 1080P TV like mine is 1920x1080.
720P = 1280x720 - the "720" is most definitely one of the #s
Keep in mind that "cheap" displays that say we are 720P, obviously have things wrong with them.
Color balance could be poor - interaction between the RGB at various intensities, contrast, gamma control, etc. etc.
Deinterlacing - if you are going to hook this to a normal DVD player or basic cable, the signal will have to be upconverted by the TV from 480i/576i to 720P for example. This unconversion operation is very much dependent on the quality of the TV.
Noise reduction - self explainatory
The best piece of advice I can give is to read reviews. You can list all specs to no end, but unless you actually "see" it, it's meaningless. Go to a store and see them beside each other and see the difference - especially if the "input" is non-HD.