It depends on the set up.
You have two main hardware specs that need to be served.
The server/streamer
The Front ends
You then need to figure out exact roles.
For the server
Rip Optical Media
Download Media
Re Encode Media
Web Server
etc.
You get the idea what do you want it to do now, and possibly in the future.
For the frontend.
Stream local content
Stream wan content (hulu,netflix,etc.)
3D support
Surround sound (dolby digital, dts, dts-hd, true-hd, etc.)
Games Roms
etc.
Also frontend Live os
poofyhairyguy has an excellent breakdown of the operating system choices somewhere around here. But if you want netflix windows, otherwise live should work fine.
Really you start simple with xbmc, then its like it can do that and that and that
.
Server hardware
Just serving up files. I like AMD for value at this price point. An Athlon X2 would suffice, but an X3 would give some more breathing room later down the road.
So really
{athlon ii, phenom ii} X2 just serving files
{athlon ii, phenom ii} X3 Just serving files, but leaving future options available.
{athlon ii, phenom ii} x4 Encoding and ripping optical media
The difference between athlon and phenom is l3 cache and you probably won't notice that. So you could probably get by with an athlon ii.
For ram 1-2GB should suffice. But there have been some pretty good deals on ddr3 out there.
HDD thats up to you
Frontend
A ion/ion2 are generally favorable. I don't know about ion2, but someone correct me if im wrong ion1 cant stream hd surround sound or do 3D.
Graphics Cards
Geforce 210 1080 hardware decode, surround sound (not hd audio)
Geforce 220 1080 hardware decode, surround sound (not hd audio), better deinterlacing
Geforce 430 hardware decode, surround sound +hd audio, and 3D support
I haven't kept up with ati since they dont work as well under linux, or xbmc live. Someone else may be able to fill you in on their options.
For a server build, im being lazy right now and going through deal forums right now
Case:
http://www.newegg.com/Product/Product.as...6811129086
PSU
http://www.newegg.com/Product/Product.as...0CX%20500W
Ram
Keep your eye open there have been a lot of sales as of late.
Mobo+CPU
http://www.newegg.com/Product/ComboDealD...mbo.581114
One thing is how many hdds did you want or plan to have is 4 enough or did you want 8 16 or 32?