Hi there,
I just built a HTPC for my parents, the spec is essentially this:
Unfortunately OCUK are currently out of stock for the motherboard and sound card, so you may want to send them a webnote asking when they expect these back in.
I included a good sound card because the onboard sound card is pretty poor (by modern integrated sound standards) and a different league from the xonar. I am connecting this up to some Acoustic Energy Aego M speakers - so the sound card is useful. But if you aure connecting it to an amp - or connecting the sound via HDMI/Optical/Co-ax - then I don't think you need a sound card - the motherboard connedctions will be fine.
I included a decent USB Freeview tuner that works with Windows Media Centre (the Home theatre software included in windows 7 that I would recommend using). There are also satellite tuners availble - just pick the USB one that you want. If you don't want/need the sound card, then you have a PCI-E slot available - so you could chose a sound card to use this - you can even get a Dual Tuner (so you can record one channel and watch another simultaneously).
The CPU /Onboard GPU combination is a potent mix - and you can play any Blu-Ray or 1080p movie file with ease.
The DiNovo keyboard is an expensive addition, but unless you want to use a larger keyboard/mouse - this is a brilliant bit of kit. It works perfectly as a media remote- but when needed it can be a full mouse and keyboard that works very well.
I would suggest installing windows 7 32bit version on this spec. It doesn't need the 64bit version to address all the RAM and Windows media centre provides thumbnail previews for videos in 32bit - but doesn't (yet) in 64bit. It doesn't make a huge difference - but as you will gain nothing from getting 64bit - I think it makes sense.