An NAS is essentially a computer in itself. Originally an NAS was just a plug and play appliance that offered a very simple, very quick way to add file storage capacity to a small business network. Usually easier than adding file servers or expanding existing ones.
Consumer NASs for home and small businesses, though, have glommed on any number of applications such as FTP, file backup and even SlimServer. What an NAS appliance like the Infrant offers is mostly just convenience in managing the RAID array contained within the device. What you're paying for is 1) RAID (a decent RAID controller in itself can be quite expensive) 2) the internal computer that runs things and 3) the software to manage the NAS.
If you just need 400GB of file storage, and have a computer that can run whenever you want to listen to music, then buying a single large drive, plus one for backup, is the better way to go. Once you outgro that and need terrabytes of space for thousands of albums, things get a bit more complicated. If you'll grow into that situation over time, though, available hard drive capacities are expanding all the time. We'll soon have 1000GB drives and no doubt 2000GB drives not long after.