this would be what I'd go for if it's purely for media watching (i.e. no tuner cards)
motherboard: Asus H61 mini ITX - http://uk.asus.com/Motherboards/Intel_Socket_1155/P8H61I_R20/
Fractal Node mini ITX case - http://www.fractal-design.com/?view=product&prod=94
silent GT640 - http://uk.asus.com/Graphics_Cards/NVIDIA_Series/GT640DCSL2GD3/
Personally, I'd stick an i5 (probably the 3570K) with a decent cooler to keep it quiet. All that's left is some DDR3 memory... 2x4GB would be plenty, a PSU, maybe 400W, a SSD and as many HDDs as you need for data storage.
I tend to go overkill for my systems though - the above system wouldn't be out of place on a high-end desktop (bar the low-end GPU). I used to take the view of trying to get the setup just right but more often than not regretted it and ended up spending more swapping parts to get it to do what I want than if I'd just gone all out in the first place! My preference is to nvidia, Intel, Asus, Corsair as they've served me well over the years and I see no reason to change, just my experience though.
To be honest pretty much any current generation of hardware will be able to play and record HD video, you could start out with the integrated gpu and if that doesn't cut it then go for an nvidia GT620 or above to handle the video processing if you encounter stuttering or other issues.
If you want tuner cards then you'll probably need to go up from a mini ITX system to a micro ATX board or use USB tuners.
CPU intensive tasks for a media PC would be advert detection, deinterlace filtering (if not using a dedicated GPU), decoding any odd codecs in software if your GPU doesn't handle them and re-encoding the video for your ipod etc so there is a real benefit to having some grunt in your HTPC but it's certainly not needed.