Heya,
Google/Wiki what RAID actually is before thinking you actually need it. And even then, again, read what it is before you actually post about it and ask something that you could have easily answered. Someone can post and say what RAID1 vs. RAID5 is, but if you have no concept of RAID in the first place, what good is that? And if you're trying to trust your data to something you just read on the internet without actually investigating it thoroughly, don't come crying when you ask what happened to it all when you lose it.
Seriously.
Google RAID and read.
And I don't mean to sound like a total internet douche or something; it's just that RAID is both simple and complex at the same time. It's not something you just jump on and spend hundreds of dollars into unless you already know what you're getting into and the risks involved.
RAID1 is for making clones of data. This is not good for `backups' unless the clones are removed and replaced periodically.
RAID5 is for uptime of data. This means, if a failure occurs, the array is not dropped, remains up, and you can recover without even shutting down. This is not a good `backup' at all. It's an `uptime' tool.
RAID is not good for backing up data. RAID is all about essentially gathering speed, increasing capacity via using several disks together, or generating uptime due to fault tolerance or redundancy. Backup is something you don't use something like a HDD for. Backup is done with tapes, optical media, online remote hosts, etc.
Consider what you're trying to backup. If it's your porn collection, pirated MP3's, and some games you've downloaded, you don't need RAID at all. Just let the disks be separate disks, and if you want to save any of it from potential loss, burn it to DVD9. If it's actual data that you simply can't lose, like financial information, business related information, work information (like digital media that you actually work with), then RAID is also not good, as it will fail eventually and again you should seek another method of backup.
NAS is basically good for sharing light streaming to multiple clients without each client needing to have a copy of the data source. At home, this is good for sharing media, like music, etc. At the office, if combined with far better backup measures, it's a good way for everyone to use lighter terminals to access the same data rather than each system be separate (and thus makes backups much easier too).
Granted, it's none of our business what you are `backing up' or `serving' through your intended NAS; but hey, food for thought.
You're likely better off just taking a machine, putting some drives in it, setting up FREENAS and not bothering with RAID. Burn important stuff to DVD9 and call it a day.
Very best,