I will try to answer in order asked.
Aspect ratios for HD are 16x9. For analog it is 4x3, except letterbox for films which vary. I have DirecTV with HD services. I currently have 1 HDTV and the rest analog. I use the HD signal for the HDTV and SD for analog sets, but will eventually upgrade to all HDTVs. People who have HDTVs without the HD service think they have HD by setting their screens to wide mode, thus stretching and distorting the images. Each manufacture differs in the way their screens set up for aspect ratio. Before buying, make sure you are not locked into 16x9.1080i or p and 720p will be the same....it's a personal preference. With a good HDTV the average viewer will not notice and you can change the aspect ratio.
When you choose an HDTV select either a Plasma or LED. Plasmas are fading out, being replaced with OLEDs. Do not judge picture quality in a store running animation. They all look great! Another factor, find one that has more that one HDMI port...you will need it for expanding your viewing habits....HD receiver, Roku or similar product, Blu-ray disc, X-Box or similar gaming console plus many other HDMI capable devices that deliver HD quality signal.
Hope this helps you...