Personally I chose to build a cheap media server based on cheap consumer level parts (power efficient, low heat, low noise, highly affordable)
I choose to use WHS (windows home server) because it's only $39.99 (cheaper than windows 7 or many others) and offers a ton of benefits you can't get elsewhere. You could dedicate an entire thread why WHS is great... and what it can do or what you can do with compared to a NAS box or Unraid.
As your media collection grows or you require drive pooling or data back up (software RAID) I would look into FLEXRAID as a superior solution.
It includes drive pooling (You can have all your hardrives show up as a single HDD locally installed) and also includes data protection and software raid parity backup. If one of your full drive of movies dies you can replace the dead drive with a new one and rebuild it. It's almost magic (really just math) that your empty replacement drive will be restored and contain everything the HDD that died did.
Biggest FLEXRAID benefit is you can remove any drive any time and slap it into any other system and read it's contents. All you data is there. That is not the case with other solutions. So if you array breaks down or your server breaks down you can lose your data. Or if you have more than one drive fail you can lose all your data.
Flexraid is not free like some others. But the good stuff in life is rarely free. You get what you pay for.
Check out my "Good motherboard for 20TB flexraid build" thread. You can see my build from beginning to end.
You can build a system as cheap as any crappy NAS box or unraid build.
Socket 1155 Sandy / IvyCeleron or Pentium CPU -$35
Asrock 77 Motherboard $69
8GB DDR3 1600mhz $29
Case + PSU $100
Antec Neo 620 watt PSU would make a nice choice. On sale $29
Tons of micro ATX or ATX cases will hold many HDD's.
I'm selling my original 10 bay tower $50 with drive cages and accessories. If you look ebay used you can get real server cases for cheap all fitted.
Even new server cases on newegg can be under $100 (rosewill is nice)
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."