So you've decided on how much space you need and the features you want. Now you should decide whether to put RAID capability on your NAS shopping list.
RAID (Redundant Array of Independent/Inexpensive Disks) NASes use multiple hard drives to provide high capacity, high availability or both. RAID also can be used to improve throughput, but this is generally not an effect found in consumer / "prosumer" RAID NASes.
RAID NASes primarily come in two or four drive flavors, although some five drive products, such as the Thecus N5200 can be found. Two-drive NASes support RAID 1 for reliability and either JBOD / Span or RAID 0 for higher capacity.
But you don't get both higher capacity and higher reliability with only two drives. RAID 1 uses the second drive to duplicate the other, so if one drive fails, you keep on running and your data is safe. JBOD / Span / RAID 0 adds the capacity of both drives together so that you see just one larger drive. But if one of the drives fails, you're dead in the water along with all of your data.
TIP: Visit either the AC&NC RAID tutorial or the Wikipedia Standard RAID levels page for quick tutorials on the various RAID levels.
Four (and five) drive NASes support the same modes as two-drive models, with the addition of RAID 5. This mode both combines the drives into a larger virtual drive and can tolerate a single drive failure and keep on running. This sounds like a great deal until you realize that RAID 5 eats one drive's worth of capacity for parity data. So those "1 TB" RAID 5 products you see advertised actually provide somewhere between 650 and 700 GB of usable space when in RAID 5 mode.
Some of the newer four drive products support additional modes such as RAID 10 and RAID 5 plus spare. But both these modes reduce available capacity even further, effectively cutting it in half. So a "1 TB" NAS running RAID 10 or RAID 5 plus spare will have less than 500 GB of usable space.
If your capacity needs don't require you to move to RAID, you might still consider it for availability / reliability reasons. All RAID modes (except for 0) can tolerate a single disk failure and continue to run. RAID 6 can even support two drive failures.
With your functional feature list done, you still might have to make a final set of choices—diskful or BYOD. I say "might" because, depending on your functional choices, you might not find products in both flavors.
Diskful NASes come ready to go with drives installed. The main downside is that when the internal hard drive dies, replacement is usually an expensive factory-only option. You also don't get to choose the drive used, but as we found in Does Drive Performance Matter in your NAS?, hard drive performance isn't a major determinant of NAS performance.
Current-generation NASes have generally moved from ATA/IDE to 7200 RPM SATA drives and faster application-specific processors such as the Marvell Orion. If performance is a key consideration (and when isn't it), you'd do well to look for products that include both these components, such as D-Link's DNS-323, among others.
Bring Your Own Drive (BYOD) products can be a good way to save money by reusing a drive left over from upgrading a PC's internal storage. They also have the advantage over diskful models of easy drive replacement since they must have the ability to format and install whatever files are needed on any raw drive.
The real benefit of BYOD is with multi-drive models, because you can start with, say a two-drive RAID 1 configuration and later migrate to four-drive RAID 5. Infrant's X-RAID, which is used in all of its products, is probably the easiest to use "expandable RAID" technology.