These different parameters are the reasons you’ll see what may appear to be very similar cards at wildly differing prices.
Here’s what I use and recommend:
For 99.9% of our shooting we need cards that are either 8 or 16 gigs in size. I’ve found 2-gig cards, while inexpensive, don’t hold enough images per card for my type of shooting. They’re not bad to have as extra cards but like 4-gig cards I’ve found 8-gig cards or larger suit me better and are still in my price range.
Now speed is where it gets interesting. You can buy a 16-gig card for under $20 to over $100 and the biggest differential is speed.
If you’re shooting images around the house a slow inexpensive card or two is all you need. But if you’re shooting air shows, action sports or fashion show runway models you’ll want the fastest and most expensive cards you can afford.
I’ve got tons of cards of all speeds and I really, really love my expensive large-capacity high-speed cards for my type of special event photography. I also love my slow, relatively cheap 64-gig cards I got from Future Shop on sale for $50 each. These guys hold thousands of images and when I’m on vacation or shooting something non-critical for web viewing (where I’m shooting JPGs) there’s no way I can fill one card in a day or even in a couple of days.
Cards come in different sizes to fit different cameras.
Most point and shoots and a lot of pro-level DSLRs shoot SD size cards which are about the size of an ordinary postage stamp.
Pro-level DSLRs use compact flash cards which are physically larger than SDs,
Sony uses memory sticks although they are moving toward SD format more and more.
There is a new type of card on the block and its one with WiFi built into the SD-size card. This is an amazing bit of technology but it comes at a price of around $100 for a 16-gig card. The big benefit is you can be shooting while automatically downloading via WiFi to your computer which can be connected to a projector so your images are being shown in real time as you’re shooting. Very cool but not cheap.
When you put the memory card into your computer for the first time format it. Always format your cards in the camera and not in the computer. A camera can just about always read its own formatting but may have issues with cards formatted in computers or other cameras.
Once you’re absolutely sure your images are saved in at least two places (separate hard drives, DVD, or online gallery) then you can format the cards in your camera and be ready for the next shoot.