Just to hit on a couple of things you are trying to figure out, with only dealing with HD Video 1080p, you will be fine with a board like the ASUS P5N7A, this uses any intel processor and has a built in geforce 9300 gpu. Most any current dual cores will meet the needs of 1080p since most of the work is done by the ge9300. The intel i7 is a bit overkill for 1080p, but will work better in the long run for sure. Again as long as your not doing heavy gaming you will not need a separate video card.
As far as the player goes just viewing HD content in .mkv format, you can use vista media center and get the cccp codecs off the internet. Also VLC is a great program too, free too.
You mentioned you are going to be using a gigabit lan which is awesome, but streaming this though wireless N to the HTPC is where you could run into problems playing 1080P content. You might see some stutter, which has nothing to do with the power of the system but just the normal latency of any wireless network in trying to get the bandwidth you need to sustain 1080p content. Any wireless network has it's downfall even at N speeds. 720p content shouldn't have any problems. Your best bet is if you can somehow get a wired connection from the gigabit switch to your HTPC. If that is not possible try looking into ethernet over powerline, I hear they sometimes do the trick.
I'm no expert in any of this, all this is from my experience, but you are in the right place to find answers! Good Luck
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