Most cost effective route would probably be to build yourself a NAS. While you can purchase them, they can be quite expensive.
I assume you want redundancy with this setup, hence your interest in RAID? If so, RAID1, 5, 6, 10, etc. would all be suitable, depending on a multitude of other factors.
One thing to point out right now, whether relevant or not, is that RAID is NOT backup. While RAID levels with redundancy (as with those listed above) do create duplicates of your data, it will not be safe from Viruses, Deletion, or other user error.
Anyways, we need some more information, such as:
- What's your [approximate] budget?
- How much storage do you need?
- Do you want redundancy?
- How important is throughput for said application?
- Are you comfortable building a rig yourself, or do you want to purchase one?
If you answer at least some of those questions, we can help you out much more.