I just went through this, and I bought 2 16g Sandisk Extreme Pros from Amazon for not quite $40 each.
It's hard to figure out what the numbers associated with SD cards mean, and what they are, and some of them are not easy to find. The size is self-explanatory, but Class 10, or worse, 100x, are not explained as real-world numbers. Both relate either to the speed that the card can be written to, or the speed it can be read from. The card often has the read speed written on the front; the read speed is usually higher, but it's less important.
Class 10 is an arbitrary cutoff where the card is adequate for video. So, it means something like "at least 10 mb/sec write speed." I'm not sure it's 10mb/sec, but it's in that neighborhood.
100x or whatever is a comparison to old hard drives that wrote at 150kb/sec, so 66x is roughly equivalent to the 10mb/sec write speed of a low-end Class 10 card.
When you take pictures, the picture data is stored in your camera's cache as quickly as you take them, and written as quickly as the camera and card can manage. Imagine that the pictures are water pouring into a bucket. The pictures stop altogether when the bucket is full, and the bucket is being emptied through a hose into a much larger tank. The bucket is the camera's cache (which is bigger for better cameras) and the hose is the write speed of the card. If the write speed is higher, the hose is thicker and the water empties faster. That means that you can get more water, more pictures, into the bucket before it fills, and that even when the bucket is full, you can keep adding pictures at a higher speed (but a slower speed than when the bucket is empty.) In this analogy, the size of the SD card is the holding tank.
With my new Sandisk Extreme Pro (claimed write speed of 90 mb/sec), my Sony a77 drops from about 8 frames per second of high-quality JPEGs to less than 2 after about two seconds. The 2 fps speed is the water emptying from the bucket through the hose; it's the speed that the camera can write to the card. If I have an older 30 mb/sec card in the camera, it manages about one frame per second, like the hose is slower so it takes longer to empty the full bucket enough for each successive shot. If I shoot in raw, the numbers are a lot lower, because raw files are bigger, so it takes fewer to fill the cache and more time to write each one.
Note that my older card is still Class 10, and both cards are far in excess of the cutoff to be Class 10. I decided to buy the top-of-the-line cards because I shoot sports and I often need a high frame rate and a short time to clear the cache. If I shot landscapes, it wouldn't matter at all. If I shot video, I'd need a card that wrote at least as fast as the video came in, but that is a lot less data than when I fill up the card with pictures. The video can still be viewed as water pouring into a bucket. In that case, the emptying hose (card write speed) has to be at least as fast as the water is pouring in, but it won't help anything if it is much faster, and even if it is a little slower, the bucket will fill up over time and overflow. I don't shoot much video, so I don't know how fast you need, but in theory the Class 10 number is the cutoff for shooting high-def video.
I hope that helps.
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/s/ Rankin Johnson IV
-Fighting for justice, but I'll settle for a reversal.
