The load of 2 simultaneous ATSC recordings is actually not very much. MythTV has run just fine on any PC with 2 or more cores I've put it on, and most new HTPCs I see for sale by commercial vendors are way overkill for just a mythbackend server. On Linux you can run the mythbackend machine headless (just command line) to lighten the load even further if its an older, weaker PC.
On a frontend client PC, MythTV has an excellent frame-precise wysiwyg video editor for your recordings, and you have the option of leaving the recordings as is with the commercials tagged so that the mythfrontend player skips over them, or you can take the additional step of mythtranscoding the recordings so that the file is actually trimmed of commercials forever and the keyframe index is reset. Again, the load on a multi-core PC is not much even when mythtranscoding.
You also have the option of running the mythbackend and a mythfrontend client all on the same PC, which is quite common and works well.
Over the years I've found the MythTV Wiki to be indispensible for hardware and configuration info.