General rule of thumb is one processor core and two GB of RAM for each video node. The system you want to build is going to depend on how many TVs you want to attach.
For OTA transmissions, you're probably looking at a couple of the Hauppauge WinTV-HVR-2250
TV Tuner cards. For whole home DVR support, you're going to need an extender for each additional TV in your home. Either an XBox or a Ceton Echo Extender.
As for your build, I can't say without knowing just how many displays are going to be attached to this system.