Depends on how you're going to use the NAS.
Two things to consider. First:
If you've got (or plan to get) a streaming device that relies on a software server, you'll need a NAS that can run this server. Then you'll have to choose carefully. Buying for something you're not going to use, is a waste of money.
If you just plan to use your NAS as storage and leave the streaming to a computer, you can use any hard drive with a network interface -- which might save you some bucks.
Second:
You need to decide how much storage space you'll need, and whether you want some sort of RAID functionality to secure your data.
In short, a single disc with network interface will give you one copy of your files, relatively cheap. A RAID box with two discs can give you two identical copies for higher security -- but at a higher price. A RAID box with more discs can give you more sophisticated and secure solutions, but at an ever higher price.