The thing everyone needs to remember today is that the USA is now using
ATSC broadcasting.
Under this digital broadcasting you don't need to encode/decode the
stream from OTA broadcasting.
So with that in mind you don't need as much "horsepower" to build a dvr.
If you have any older hardware that will use the old NTSC like a vcr or
cable box and you want to capture that source then you will need an
capture card with a ntsc input.
If you want to use Directv/Dish Network then it gets a little more
complex but depends on what you would like to use as far as the output
from the reciever.
Most good capture cards have component and s-video inputs as well as 1
atsc and 1 ntsc input for ota/qam capturing.
You can acquire capture cards for just component input but most don't
have good capture quality that I have found.

Do a google search for HTPC or PVR and you will get alot of hits for
building a pvr but some are older and refer to the older ntsc format.
Under the new atsc the stream is already in mpeg format so your just
essentially copying the stream to your drive.
Under Linux there is MythTV and Freevo for frontend/backend pvr.
MythTV has pretty good documentation on there website.