I wouldn't recommend putting your music on the main internal drive, CT. (I'm running a Mac Mini with a 4TB external FireWire drive.)
First, you'll have contention on the internal drive between system functions and serving up music. How much of a problem? I don't know, but it should be considered.
The external drive makes backups very easy. I have another, similar drive, and simply clone the drives after new music additions are made. The clone goes in the fire safe.
Given that, I don't think you need RAID, although it would be handier than my solution.