Okay, since you're not gaming here's what I would do:
Video card: I'd go with a Radeon 9600 non-pro like this one:
http://www.newegg.com/app/ViewProduc...alog=48&depa=1
I use this video card in my htpc & it works great.
Sound card: Either an Audigy 2 or M-Audio Revo will give good bang for the buck - both will pass a digital signal for the future receiver (see the mobo line for more comments on sound). One thing that may matter to you is whether the card has SPDIF out (via either coax or optical). (This goes for the mobo solutions listed below, as well.) If not, you'll need a 1/8" jack adapter to make it work with your receiver. Of all of the sound solutions listed here, the only one that I know of that has this is the M-Audio Revo.
Mobo: This is a tough one, because almost any will do the trick. If you want to do an integrated sound solution, that will save you a PCI slot & if you get the right one, will pass a digital signal fine. A couple of good choices would be:
http://www.newegg.com/app/ViewProduc...=BROWSE&depa=1
http://www.newegg.com/app/ViewProduc...=BROWSE&depa=1
http://www.newegg.com/app/ViewProduc...=BROWSE&depa=1
As far as the mobo decsision, it really doesn't matter that much as long as you get a good stable board. I own the last one on that list & use it for my pc that I work on - no problems with it & it's a real nice board to work with. Lots of people use the first board (the Asus) because it's got a really, really good sound chipset on it - it will do onboard digital decoding (*I think* it's the only sound chipset capable of that). Can't comment on that second board, but Epox makes good stuff so there shouldn't be any problem with that one either. All 3 will pass a digital signal, so you should be good to go.
I can't comment on the HD card since I have no experience with them. As far as hard drives go, try to stay with Seagates or Samsungs. The Seagates have been proven to be the most quiet & coolest running drives, & the Sammies IME aren't far behind. You'll probably find better deals on Western Digitals & Maxtors but you'll sacrifice noise & heat for the price. Just my .02 on that topic.
As far as a case goes, the best bang for the buck would be the Antecs. If you stay with a micro-atx board, you can look at this case:
http://www.newegg.com/app/ViewProduc...=BROWSE&depa=1
These are full-atx cases:
http://www.newegg.com/app/ViewProduc...tby=14&order=1
http://www.newegg.com/app/ViewProduc...=BROWSE&depa=1
On the Silverstone link, scroll down a little for the desktop cases. Also, the Silverstones don't come with power supplies, so keep that in mind.
As to the DVI cable question, I don't think 30 feet is too long, but I could be wrong about that. Here's a link for a list of free software - there are some HTPC essentials on there, & some fun stuff:
http://www.avsforum.com/avs-vb/showt...=free+software
Hope that helps.