I keep starting a reply and never finish so hopefully here is a good reply.
Currently at home I run a media server that hosts movies, tv shows, mp3's, flac's and pictures. I do not use this machine for anything else other that as a media server. Don't even have a monitor, keyboard or mouse hooked up to it. I view/listen to the content on it via my modded Xbox and a laptop.
Here are the specs on it:
Case -
http://www.newegg.com/Product/Product.asp?Item=N82E16811129158 The Antec case I use is very quiet and holds a ton of drives.
CPU/Mobo - Intel P4 3.0 / Asus Mobo - I got these together as a combo at my local Fry's for like $150. The mobo has onboard video/sound/LAN/SATA. It also has 5 PCI slots as well as an AGP slot.
Memory - 1GB of Corsair Value Select - It was on sale and since I don't do anything other than transfer files on this machine it was kinda of overkill but I got it super cheap.
Drives - As of 5/8 I have 1 main drive that holds the OS (40 gig) that is plugged in via the onboard PATA slot. For all of my data I have 9 Maxtor 300GB DiamondMax 10 SATA 150 drives as well 4 Maxtor 300GB DiamondMax 10 SATA 300 drives. SATA 3 is the same as SATA 300 btw.
Controller cards - I ONLY use drive controllers made by Promise Technology. (
www.promise.com) They just work. I have tried a couple of others like Adaptec and whatnot and now will ONLY use the Promise cards. Super easy to setup and run great. They are 4 port SATA controllers. Two of them support SATA 150 and one supports SATA 300. The cards are PCI cards btw.
Audio card - I use and love the Echo Media brand of cards.
OS - I am using Windows XP. I had consider Server 2003 but decided it was overkill.
Display - The wife and I just bought a 50" Plasma a few weeks ago and have a DVI connection between the server and the tv.
Software - Currently I am using SnapStream BeyondTV 4 on a demo until I can find a key or a cheap retail version of it. BTV4 is similar to a Tivo. I can watch save videos as well as watch/pause/record live tv. This may not be what you are looking for but thought i'd mention it.
A few notes on cooling. For the most part anything over what is provided with the case is overkill usually. If you think you need extra cooling download a program that will give you temperature readouts of your cpu, drives, etc. You can find one of these programs for free on download.com or somewhere similar.