yes ntsc is dead unless you still have cable that hasn't switched to hdtv yet
many of the newer hybrid cards will do either, in your case atsc is all you need
mythtv if you use something like mythbuntu is basically plug and play, simply start with suitable hardware and put in an install disk and go from there
setting up mythtv is a bit complicated cause it can do an awful lot but for the most part the setup menus are pretty self explanatory and the documentation at mythtv is pretty good
couple things to consider:
disk bandwidth, do not skimp on it, hdtv sends several megabytes per second at your hard drive, use SATA and get a sizable disk and if possible use two with a dedicated disk for recording onto
you may very well re-encode most recordings to save space, mythtv will do that and flag commercials, just keep in mind it takes some serious cpu power to do this with any speed so if you build an atom based htpc keep that in mind as the performance will suck and hdtv files are damn big to begin with
I also recommend a dual tuner or a pair of them, it helps if you can record one channel while watching another.
don't skimp on a tuner card, make sure it has hardware mpeg2 encoding
another new option is a network tuner that connects to your htpc via ethernet, mythtv will work with those and has the added bonus of working on all pc's on the network
they basically stream the video/audio to whatever is setup to receive it right over ethernet
you can also play back from the mythtv box over ethernet by simply setting up samba to share the recording directory on the htpc, that is how I do it
I have been using mythbuntu for a few years now with few issues once it was set up.