PAL and NTSC are commonly used to refer to two different things - the color encoding in use, and the vertical refresh rate. In the past, these were tied together - NTSC always ran at 60Hz (59.94Hz with color) and PAL always ran at 50Hz. NTSC is still like this - while technically possible, there is no 'NTSC-50'. However, in recent years, there is such a thing as 'PAL-60', which is the PAL color encoding but at 60Hz.
The only main difference between RF and composite+audio is that both the video and audio are remodulated to different frequencies (and due to the presence of the audio, the video is a mite bandwidth-limited as opposed to a composite signal). All analog video signals (in terms of standard low-scan devices and not custom stuff like EGA monitors that run at some weird line rate) run on either PAL or NTSC (SECAM is basically a variant of PAL, and is pretty much nonexistant outside the countires that primarily use it). Even RGB is this way, although there is no color encoding, and as such PAL/NTSC only refers to the vertical refresh rate.
HDTV itself is pretty much useless for the Amiga, since it is a transmission method. All HDTV sets are primarily 'monitors', with some that have an internal HD tuner. Such monitors tend to have any of the following to support the HD resolutions: analog component, VGA, DVI, HDMI (which is compatible with DVI devices with an adaptor). Any system will work with any HDTV if it supports one of the HD resolutions (of which are 960x540, 1280x720 and 1920x1080) and the signal is close to the HD standards for each mode (540p and 1080i are the same, I'm unsure what 720p is).
And with an adaptor, you can use an RGB SCART cable with the system of your choice to turn it into low-scan analog component, which would presumably work with any analog component input.
