there are 3 types of HD, 760×480, 1240×720, and 1440×1080
standard is 704×396
there are 2 methods of HD, I and P
once encoded the I and P get stripped from the signal and all you have is the resolution. now once it is encoded it depends on the hardware and the options the encoder chose… you could end up with a SD file that is better than a 720 HD file.
while higher resolution usually means bigger file size and better picture quality, its not always the case because it depends on the encoder.
computer CRTs have been “HD” since about 93-94, projections since about 95, and plasma/lcd since about 2002-2003.
for LCD its a little different though. they have manufacturer reccomended resolution and that is the only resolution you should be running them at. so if your computer lcd has a resolution of at least on the 2nd of 768 and the first higher than 1024, its a HD monitor. all your home displays as long as they say they are HD or have a HDMI port, it can take a HDMI signal without loss.
for a clean encode a SD or 480 HD will be about 400-600, 720 will be about 1.1-1.3g and a 1080 will be about 1.6-2.2 for a clean encode. the bigger the file is, the higher the chance its a cleaner encode. thats what determines how good the quality will be. thats for a roughly 1 hr show.
and even with those guidelines you can have a 720 1 hr show that is in the right range, but still doesnt preserve the HD quality because of the encoder/codec used. its just a higher chance to be a good encode.
now weather or not you can play the file depends a lot on your hardware for the PC/laptop. faster RPM for the drive, better video card for the rendering, and better CPU to proccess the encoding codec are important. if you lack in any of those 3, you wont have good performance.