I have got a setup similar to what you are planning... My goal has been to be as thrifty as possible so most of the hardware is hardware that I have scrabbleed up or at least got cheap.
I have the media pc set up for the past 12 months and got it to a place where I am happy with it... I wont bother going through the full setup but (happy to if you want more info) here is the overview of the PC's
Media PC
Compaq p 800, 384mb ram, 1x20gb HDD (OS), 1 x 80gb HDD, 1x120gbHDD, generic 128mb video card, sb live 5.1 sound card, 100mb on board network card, Leadteak TV card, streamzap remote, IR keyboard with built in gamepad like mouse, BenQ projector
Software
WinXP Pro, Meedio
http://www.meedio.com, power DVD, various utilities
obvious question: why meedio? First reason $$. Meedio is cheaper than Win WCE and I do not have to have "certified" hardware to run it on. EG the leadteck tv card is only software encoding so not supported by MCE.
Why not Linux? Fraid I am a windows boy, windows support for a job so much more confortable with windows.
where is the NAS? Good question... I have just set it up over the past month so still going the hardware collection as the PC (AMD 700) only has 20gb hdd. But I am currently using the nas to backup my photos and home movies to.
Streaming from the Nas? I have tested this currently with excellent results. MP3's are certainly not an issue. DVD video ripped with DVD shrink is not an issue, flawless playback. MPG (Dvd quality home movie) I played 2 videos back to 2 PC's (media pc and office pc) at the same time with no stuttering or skips.
With the new version I copied a 7gb capture from my raw video camera. As a comparision DVD quality id 4GB per hour, raw video uses 12gb per hour. I streamed this to my pc... there was a bit of stuttering at the start but this was reduced to almost nil after about 10 sec.
Things to do
• Get the disks from the media PC into the NAS
• Upgrade the media PC - although it runs fine there are a couple of times when backgroud tasks cause the video playback to have a very small stutter. I notice it but no one else would and it is a little noisy
• Get a hardware encoding TV card
• Get a second meedio PC for the TV in the family room
• MORE DISK!
And the big test... what does my wife think Ok lets be honest at first she though it was a huge waste of time and she did not want a PC in the lounge, now she is quite happy with it and in fact with meedio my older kids (6 year old twin girls) can even work it themselves to get their own music
So my advice would be to check out meedio as an option for the htpc but certainly worth it