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If you're building for freeview, you need a dvb-t tuner card. The good thing is that Freeview comes down the aerial by and large, but it is compressed bandwidth, so you will get some artifacts and macroblocking in practice. Also, your tuner card can only decode one stream at a time, unless you buy a dual tuner, which you really should if for example, you wish to watch one channel whilst recording another. As for audio output, you can connect your mobo output to your stereo, assuming your motherboard allows onboard sound, but remember you will get higher quality audio from a dedicated soundcard - I wouldn't buld without that, I also use a graphics card as I find it superior to onboard again, but I do go for passively cooled - noise is everything on a home theatre pc.
You can pick up a blu ray drive for around £70. I upgraded my drive to an LG one & I like it a great deal. backwards compatible of course with dvd & cd, but to run Blu ray on windows seven, you will need additional software - I use power dvd 9 for mine & it works pretty faultlessly.
Bearing in mind that you will need expansion cards to get the best from a media pc, i would strongly recommend that if you're building from the ground up, be very rigorous in your selection of motherboard. You can get away with a reasonably modest processor - mine is an E600 dual core intel number - old hat by today's standards, but fine for a non gaming box. However, what you don't want is to end up compromising your tuners/sound cards etc because your mobo doesn't have the slots you need. Think carefully on your cards & go from there.
Also, the nature of media boxes means that you will be usb port hungry. One for a wireless keyboard, one for the remote IR blaster and a couple for media use. Mine has four & they're in use every day, two permanantly, so be sure to have enough for your needs; The box in the picture also has a firewire dv port on the front - great for importing video from a camcorder - it's all in the planning.
For the record, my cards are:
Creative xfi xtreme music
Hauppauge Nova T-500
XFX 9400gt silent
Hauppauge HVR1700 for Virgin analogue cable box input.
HDMI is the way to go. If you're setting up in a living room/theatre room scenario, and I assume that's the point of it, then you need the microsoft remote which is very good, comesd with IR blaster & red eye for talking to cable/satellite boxes.
I also use a wireless keyboard and mouse, so essentially I loaf about on my backside typing, surfing & doing all the media bit without moving.