Nairi:
HDMI = High Definition Multimedia Interface
Blue Ray has a definition of 1080p .. That's 1080 lines of resolution, with a progressive scan technique. This allows for a picture that does don't jitter or flash as much when your eye moves across the screen. A BR payer will downconvert to 720 if need be, but there is no going back in my opinion. When HD2 comes out in a few years, you will double the 1080 res.
I suggest getting a HDTV with a 1080P spec., HDMI inputs, and optical outputs. Stay with LCD and not plasma... there are issues with plasma.
Also to do this right, get an A/V receiver and use a 5.1 speaker set up. It is the only way to watch movies nowadays, otherwise you will be missing a lot.
If you need more help let me know, or email me directly.
Aram