Easily the most important accessory to any digital camera -- and yet one of the most overlooked -- is the memory card. Memory cards store your pictures and videos, and can have an effect on how fast your camera operates. They're the digital equivalent of film.

A memory card should be considered a mandatory purchase alongside any new digital camera. Cameras these days rarely come with an included memory card and, if they do, it's usually too small to take more than a handful of pictures. Merchants will often offer a package deal with new cameras, tossing in a memory card and camera bag for an extra $25 or so. That's usually a good deal, but it pays to know what kind of card you'll get in a bundle like that.

Memory Stick, XD picture card, SDHC, MMC, CompactFlash and a seemingly unlimited amount of variations on each make the memory card market wholly confusing to the uninitiated. Luckily, things have been made much simpler recently. Let's take a look.

What Type of Card?

The first and most important factor in choosing a memory card is the type that your camera will support. The most common formats, by far, are SD and SDHC. Almost every new camera from every brand supports these formats. (The difference between the two is capacity. The largest SD cards hold 4GB, while SDHC cards store anywhere from 4GB to 32GB of data.)

In short, if you're buying a memory card for a brand-new camera, buy SDHC. It can hold more pictures and videos than SD. And if you change camera brands in the future, you can still use your old SDHC card.

There's a slight chance that your new camera might only accept MicroSD cards. Samsung's DualView series only accepts MicroSD, as does the Casio G1, and a handful of others. This is a common format for mobile phones, but not so much stand-alone cameras. Check the specs and manual to be sure. The good news is that if you're forced to go with MicroSD, the cards usually ship with an adapter that allows you to use MicroSD in standard SD/SDHC slots.

If you're buying for an older camera, things get slightly more complicated (you can skip to the next section otherwise). If you have a Sony, Olympus, or Fujifilm models released prior to this year, check the manual to see which format you need. Your Sony might require Memory Stick, and your Olympus or Fujifilm might require xD (though many of these cameras shipped with adapters to accommodate MicroSD cards in an xD slot). For other brands, if the camera was released prior to 2008, you probably need SD – older cameras don't typically support SDHC. When in doubt, read the manual.

It's also worth mentioning the CompactFlash standard. You probably won't run into this format unless you buy an expensive professional or semi-pro dSLR, so don't worry too much about this one.

How Fast?

The next factor to consider in your memory card purchase is the speed of the card. (Given the number of formats, there are a ton of variations. So for simplicity's sake, I'm just going to discuss SDHC here. If you need a non-SD format, consult your manual or the camera's specifications before purchasing a card.)

An SDHC card's speed is based on its “class.” SDHC cards come in Class 2, Class 4, Class 6 and Class 10 varieties, indicated on the front of the card in a three-quarters circle.

The class reflects the guaranteed minimum write speed: In short, a faster card will allow your camera to work faster, or at least as fast as the camera is capable of going. A properly functioning Class 2 card will never write data at a rate slower than 2 MB per second. A Class 4 card will never transfer data at a rate slower than 4 MB per second and so forth for Class 6 (6MB/s) and Class 10 (10MB/s) cards.

As usual, your camera's manual is your best direction here. Most Class 4 cards are capable of recording HD video, but some cameras refuse to even try if they detect that the card is only a Class 4, regardless of quality. If the camera manual says Class 6 or higher, bite the bullet and buy the nicer card—you won't regret it and it really won't cost you much more.

But, as a general rule of thumb, a Class 4 card should provide ample speed for the majority of all compact cameras. But if you're shooting with a camera that's capable of high-speed burst shooting (greater than three shots per second), HD video at 1080i or higher, or a dSLR, you'll be better off with a Class 6 card or higher.

How Big?

The last factor to consider when buying your memory card is size (or storage capacity, to be formal about it).

As mentioned, SDHC cards range from 4GB up to 32GB. Here at Digital Camera HQ, we think that, in general, it's better to buy a few small cards rather than one large one. Having a few small cards might be less convenient, in the sense that you'll have to swap cards in and out more frequently. But it will prevent the worst-case scenario: storing 30GB worth of pictures and having the card fail or go missing.

Generally speaking, a 10-megapixel camera will be able to store roughly 1000 JPEG pictures on a 4GB card. A 12-megapixel camera can store roughly 810 JPEG pictures on that same card. If your camera supports the RAW format, that number drops significantly, since the files are uncompressed. Elect for a larger card, in that case.

If you're planning on using your card to shoot any video, go for at least an 8GB card. It's impossible to say for sure, given the variety of compression codecs out there, but a good rule of thumb is that an 8GB card can hold about 90 minutes of good-quality 1080p HD video. Most cameras max out at 720p, so that's significantly more time, and cameras that shoot in VGA/standard definition can store much, much more on an 8GB card.

Bottom Line

Most casual shooters will find that an 8GB Class 6 SDHC card will be the only card that they ever need. It has room for more than 1,000 pictures and plenty of video recording time to boot, and can write as fast as any camera needs. These are readily available for about $20 each online, so buy a few while you're at it.