Form Factor
The vast majority of SSDs come in the 2.5-inch form factor rather than the 3.5-inch size you'll see for most hard drives. (This means that SSDs can be used in laptops almost as easily as desktops—a nice feature.) But practically all drives also come with a bracket that will let you put the SSD in a 3.5-inch drive bay; exceptions to this are rare enough that reviews will usually point them out when they occur. In addition, many new cases, particularly those designed for gamers and enthusiasts, come with built-in brackets specifically for housing SSDs. One way or another, you're probably covered. If you're buying an SSD for a laptop or for a very tiny nettop, you may also want to pay attention to its thickness: Most SSDs are between 7mm and 9.5mm thick, but some can be as thick as 12mm—and they might not fit in really tight enclosures.

Just to be clear, no SSD is a bad performer. Even the slowest and least expensive SSD will melt the metal off of average and above-average hard drives. But that doesn't mean that all drives are created equal. Two SSDs, even if they both use the same version of SATA, can differ wildly in performance. This is because of the SSD's controller, or the chip that manages the movement of data onto and off of the flash memory. Some controllers are just faster than others when working with certain kinds of data (usually either compressed or uncompressed). Random access rates, which come into play for general OS duties and when you're running other programs, can be even more important. These are measured in IOPS; for both read and write speeds, 40,000 IOPS or more signals a particularly efficient drive. Keep in mind that you won't see these speeds in every application, but knowing what's possible will help you find a drive that's a good match for you. Look for information about data types and IOPS in reviews to get an idea of what you're in for with any given drive. (Note: For reasons related to their architecture, lower-capacity SSDs in the same series can actually be slower than higher-capacity drives under certain circumstances. Reviews will usually point this out when it occurs, too, but just remember that 128GB and 256GB drives may not run at the same speed, for example.)

Other Important Features

TRIM support.
You've probably heard before that when you delete a file, you don't really delete it—you just prevent the drive from being able to find it. This is as true with SSDs as with hard drives, but an added wrinkle comes from the SSD's natural inability to permanently delete files manually—this can cause serious performance degradation over time. TRIM is a function of most current-generation operating systems that helps SSDs know what files can be permanently done away with, with no intervention on your part. Most new drives do support TRIM, but it's definitely something to look out for.

Lifespan and Warranty.
SSDs have a finite lifespan; flash memory can only be written to a certain number of times. (This is also why you want to never defragment an SSD—you'll expose it to a lot of additional wear and tear in a short period of time.) Knowing how many operations you can expect your SSD to endure may affect your buying decision, so if you're concerned that heavy use may cut down the life of a drive (usually about five years or so), look for one rated for even more. Many SSDs also come with extensive warranties (often for three years or more), which can add peace of mind.

There are two different kinds of flash memory technology you may encounter when shopping for SSDs: MLC and SLC. MLC (or "multilevel cell") stores multiple bits per cell; this is the more common and less expensive type, but it can also be rewritten fewer times. SLC stores only one bit per cell, which makes drives that use it cost more—but they're also faster, more reliable, and longer-lasting.

You can arrange SSDs in RAID configurations, just as you can with hard drives—assuming you have the money for two or more SSDs. Options include "combining" multiple smaller drives to create one big one (RAID Level 0, or "striping") or "mirroring" one drive onto another for added security (RAID Level 1). All you'll need is a RAID controller, something many motherboards today have built in. RAID is a complicated subject beyond the scope of this article, so to learn more check out our story, "RAID Levels Explained."

Our Recommendation
There's absolutely nothing like an SSD, and once you've booted into Windows with one chances are you won't want to go back. If you're ready to take the plunge, find a comfortable but small-capacity drive—128GB is a good size—and pair it with a spacious hard drive (or two) to give you plenty of room for all your non-application storage. If both drives support 6Gbps SATA III, you'll realize the full benefits of the speed at your disposal. Regardless, this setup will give you an excellent overall balance of price and security—and performance that must be experienced to be believed.

For news and reviews of the latest in storage devices, check out our Storage Devices Product Guide.