You won't get HD Channel if your cable box don't output HD on its COAX output called CLEAR QAM. Some cable box produce HD output on its HDMI output only and no CLEAR QAM on its coax output.
Try HD Antenna and you will get HD channel from local transmission calles ASTC
I have 2250 and i get ASTC from Air and CLEAR QAM from cable.
1080P is 1920x1080 pixel resolution @ progressive scan mode. 720P is 1280x720 pixel resolution @ progressive scan mode. 1080i is 1920x1080 pixel resolution @ interlace scan mode.
Depending on the sensitivty of your eyes when watching HD movie you won't see much difference until you watch it on a 42 inch HD TV.
When using an HDTV or LCD as monitor as computer monitor 1920x1080 has a lot if desktop space compare to 1366x768 and the difference is extremely noticeable.
So 720P HDTV are not good computer display. Very limited desktop space.
see path below http://en.wikipedia.org/wiki/High-definition_television