Questions about DVD-movie format

DVD-movies vertical resolution is 576 pixels for PAL and 480 pixels for NTSC But what about horizontal resolution when stretching or zooming to common 16:9 TVs? Does resolution differ if HTPC does the stretching instead TV? I mean target resolution because source resolution is always 720 pixels. Some say it's 1024 (missing pixels are interpolated) for PAL but some say it's still 720. Why anamorphic DVD-movies larger than 16:9 aspect ratio have been also letterboxed? For example 2.35:1 anamorphic movie in PAL case has only 435 vertical pixels for image and rest are black.

the horizontal resolution (at least for CRT TVs) is meaningless. on a DVD it's either 720, 704 or 352 pixels, but this says nothing of aspect ratio. if you're thinking square pixels, then you figure it out from vertical resolution * AR, so for PAL 4:3, you get 576*(4/3) = 768, and for NTSC you get 640. what comes out of a HTPC really depends on the setup - it could just be outputting 720, or it could be outputting something else. with a CRT television there's not going to be much gain above 720 unless you have an extremely good TV (not even the special-for-monitoring ones show much difference).

But what about when using RGB signals on a CRT TV, and not PAL or NTSC?

Whether it's component (RGB, RGBS, RGBHV, or YPrPb), Y/C (S-video), composite, or digital (SDI or PDI) it is still either PAL or NTSC. These standards define the timing and resolution. (SDTV - of course HDTV is different, but can be conveyed in all the above forms)