[GroundUrMast]

I would suggest you keep things simple to start with. Hook up the antenna to one TV with an RG-6 cable. You can run it temporally to start with, though doors or windows. Be sure the TV is set to scan for 'Air' or 'Antenna' channels, not cable or 'Auto'. Any TV built after the first third of 2007 has an ATSC capable tuner, the FCC mandated that manufactures had to include ATSC if any tuner was in the TV. (ATSC is the standard for broadcasting TV over the air in digital) You'll then know what signals are reliable and if the antenna aim needs adjustment or the antenna needs to move out of the attic.

If your TVs don't have ATSC tuners, it suggests they are pre-2007, standard definition (480i resolution). If that's the case, outboard tuners (digital to analog convertors) make sense, otherwise I question the economics of buying a bunch of tuners that only duplicate the built-in capability of all newer TVs. For the cost of the tuners you should easily be able to hire a structured cabling company to run RG-6 to each TV location.

Your situation is a bit unusual with HDMI and CAT-5 cable run, but little or no RG-6 coax. The traditional method of distributing OTA, cable and satellite is coax. To make channel surfing easy for the wife and others, I would cable from the antenna to a splitter. Then from there, a coax to each TV. The viewer simply needs to use one remote, the original remote shipped with the TV.

I own several SiliconDust tuners. I use them as a pooled resource, available to any PC to use. The media center software should be able to 'lock' a tuner while it's in use, forcing other PC's to search for an idle tuner. The current HDHR-3-US has two tuners, each independent of the other. They connect to the antenna using F type coaxial connectors just like the tuner in the TV. They connect to the Ethernet LAN using a standard Ethernet patch cable connected to an open port on your Ethernet switch/router. When the HDHR is powered up, it asks for an IP address from your router the same way your PC iPhone or other network devises would. When viewing a TV program broadcast in 1080i at full rate, you can expect to see data at rates up to 18 Mb/s from the HDHR tuner to the PC that decoded the MPEG-2 video+audio stream. Wired Ethernet is far more reliable than wireless. If I am going to record to hard-disk I avoid using wireless and use wired Ethernet from the HDHR all the way through to the PC that's doing the recording. If you run many HDHRs, and have other high bandwidth traffic on your LAN, a Gigabit Ethernet switch would be recommended. A 100 Mb/s Ethernet switch should be able to handle four 1080i streams if no other significant traffic is on the LAN.

A home theater PC is the interface between the home Ethernet LAN and the TV. This is an area of nearly unlimited options and choices. If you want to stream Netflix, Hulu, etc. Windows 7 Home Premium is likely the OS of choice.