I have been working with computers professionally for 20 years and I have to agree with Kthomas in that RAID might be overkill. I would consider external USB or Firewire drives for a cost effective approach. Step up to the next level - there are several good NAS boxes out there that just hook into a home network (use Ethernet) Buffalo Technology has a "Teraserver" that is pretty slick. I have installed two so far for clients. You can RAID/MIRROR or just run em raw. WEB interface ... good for non-network people.
If you go FULL on ... 3Ware SATA RAID controller - I could go on and on why I love these controllers but lets just say they are fast and work well in any type of environment. Dont screw around - get a server case and something that hot-swaps fans as well as drives. Make sure you get a good solid power supply / supplies. Get Seagate drives or comparable with a 5 year warranty and keep a spare or two. If youre just streaming music I wouldnt worry too much about drive speed. It will make noise find a safe temp friendly place to put it. Run a network OS on the box and BUY the OS and application software. Server 2003 is a bit of overkill but it has some nice tools built in. Your looking at some serious money but it will last a decade or better and you can expand to your hearts content 2,4,8 Terabytes whatever you need just add drives and controllers.
http://www.newegg.com/Product/Product.asp?Item=N82E16816116029
I hate to harp but :
BACKUP/BACKUP/BACKUP !!!!!! You get started collecting music and pretty soon you will have stuff you dont want to loose. Drives are cheap. Dont get caught with your pants down.
Computers HATE people