Welcome to the forums Dman78!
Have you checked into if any of your lcd/plasma TVs have media sharing built in. As for the tube tvs, you will need a DSM-750 for each TV(I believe), although I'd look at the Western Digital Live products too.
If you want to watch TV on the media PC or any PC in house and have any of them function as a DVR I'd stronly suggest not getting a TV tuner card (my big mistake) for the media PC; instead get a HD Homerun from Silicon Dust HDHomeRun (US/CA) « Welcome to SiliconDust
As for the build itself... any of todays dual/quad/sext core CPUs will be fine. No need to cram it full of RAM 4GB is fine in mine! The biggest expense in a Media PC will be storage and the video card. I'd suggest a card with HDMI (full size not mini) out!!!