Interesting thing about 1080i vs 1080p, in the world of LCD, DLP, and plasma. The resolution of both formats is the same, at 1920x1080. The difference, whether it's interlaced or progressive scan, is supposed to be where the difference in picture quality comes from. interlaced, which is typical of TV signals for decades, sends one half a frame of picture at once, then goes back and scans the second half. Each half is called a field, and normal NTSC TV delivers 60 fields per second, giving you 30 frames per second. Progressive scan delivers the entire picture at once, offering up to 60 frames per second. (for comparison, traditional film delivers 24 frames per second).
Why is this interesting? Interlaced is a necessity with tube type TV sets, as the process to deliver a picture in a TV tube is an electron scan that delivers the picture line by line. LCD/DLP/Plasma sets ALWAYS deliver the image progressive (i.e all at once). meaning, they convert all interlaced input to progressive input. So 1080i is always converted to 1080p in the set. What does this mean? Even a 1080i DLP/LCD set is delivering a 1080p picture. And a 1080i signal (the highest current broadcast signal) is delivered to your eyes as 1080p.
Lastly, your source input is the most important part of the process. Even on the older HDTV sets, the input stream makes a huge difference to picture quality. Composite cables are better than a standard connection, and hdmi is the best yet. And if you are watching standard cable/dish/broadcast on a 50" or larger HDTV, be prepared for a really crappy picture. I mean fifth generation VHS tape copy bad. You'll be blowing up a 640x480 image designed to be seen on a CRT to huge digital sizes, where you can see every pixel from across the room as though it has really bad jpeg artifacting. You will not like watching any old VHS tapes, and even older DVDs will lose their luster. 
