Well the "OS" you use will directly impact what hardware you need. unRaid requires minimum to no CPU cycles. You can use an old celeron as your processor with no problems. RAM is a little more important but not by much. It will help if you are doing things like running multiple pre-clears (Drive initializations) at the same time or a lot of plugins.
What is most important for any of these are things like onboard NIC speeds and the number of SATA ports. Especially ones that get the maximum throughput in your drives. If you have a ton of SATAIII hard drives, having SATA III ports would be a bonus. If you only have SATA II hard drives, well you dont need as many ports. For additional SATA ports, it doesn't matter if you use a RAID controller or a SATA controller, but you won't use any of the features of the controller, you would just have it in JBOD mode.
If you want to actually use a hardware raid, then programs like unRaid, FlexRaid, etc. (Which is the topic of the thread) aren't necessary. You would just run a simple linux kernel distro with your array shared.
unRaid has a system builder FAQ as well as forums explaining hardware that people have used that has worked for a lot of people. My CPU is way too powerful for my unRaid server but I had it sitting around with MB and Ram so that's what I used.
Say you use unRaid and set up your array. Once you create your Share, you just map the hard drive or go to it from any windows box on your network and write files to it. Playback is just as easy. Map a drive letter to your \\\ ower\\movies or \\\ ower\\mp3s or whatever your share is, and then just add that as one of your media browser sources. If you have gigabit ethernet, you shouldn't have any issues with direct playback (Assuming your client is setup and configured correctly for normal playback of course)
Two of the most popular add-in cards for unRaid servers are both made by Supermicro. Both are like $100 each
PCIe (newer MBs have a lot of these slots, but they require 1-> 4 breakout cables)http://www.newegg.com/Product/Produc...82E16816101358
PCIx (Or legacy PCI) (8 onboard SATA ports, fits in legacy PCI slot)http://www.newegg.com/Product/Produc...82E16815121009
A lot of people like these NORCO rackmount cases (For 20 sata hot-swap bays)http://www.newegg.com/Product/Produc...82E16811219033
But you can easily get a normal full tower with 5-3, 4-3, 3-2 hard drive cages to fit into the front 5.25 bays. With a system like unRaid, which runs on a USB thumb drive, you don't need to waste a bay on an OS hard drive.
In ANY media storage server, make sure you get a power supply with a SINGLE +12V rail, and not a split power rail. Since you will be using almost all of your power for hard drives and not for things like video cards you want to make sure you are able to use all power on any given peripheral cable.