All of these suggestions sound good on here but nobody has really considered this scenario (unless you already have this sorted)
You have your iTunes library on your RAID 5 configuration, everything is running smoothly for the first few months,you wake up one morning and overnight the RAID controller died and you've lost all your data regardless.
Point I'm getting to is that it's all well and good having a NAS but this should be backed up with 2 drives rotated weekly (at the very least) so one stays off site at all times. At worst you lose 1 weeks worth of data.
Just a thought doesn't disregard all the other suggestions. Remember that RAID/NAS etc isn't a backup solution.