I don't need to tell you that you need to entrust your photos to a good quality SD card. It used to be that I might recommend slower cards as being sufficient for your digital camera, reserving more expensive higher speed cards for multi-channel digital recorders and the like. But today most DSLRs have burst mode shooting that might suffer in throughput if inferior cards are used. If you find that your camera takes a long time to process a bunch of images, your SD card might be the problem. (Though such performance is also a factor of the internal processor, the memory pipeline and how much buffer memory is internally allocated to this function.)
Cards are rated in two different ways. First there is the Class, which specifies the minimum sustained write speed in MB/s. Class 2, 4, 6 and 10 cards are available. Second, the x rating compares the transfer rate of the card to the CD-ROM drive speed of 1.2 Mbit/s. Don't ask why, just accept it!
Cards invariably have faster read than write speeds. Thus, though it is not specified which operation the "transfer rate" should refer to, it is invariably the read speed. Note that one needs to divide this figure by 8 to get units of MB/s, so one can compare directly to the Class rating. Common ratings are 40x, 66x and 133x, corresponding to 6, 10 and 20 MB/s respectively.
In an ideal world one could use these two ratings to learn something about the performance of an SD card. A 133x Class 6 card could be assumed to have minimum read/write speeds of 20/6 MB/s. Unfortunately no organisation checks these ratings to hold manufacturers to their claims. So we must instead take them only as a guideline, relying on technical reviews and tests, as well as practical field experience, to find the best cards.
It would be easy to say "get the largest fastest card on the market". But such a card would also be very expensive, and might be overkill for your purposes. There is something to be said for carrying several smaller cards. If one fails, you lose fewer images. It's the "eggs and basket" dilemma. I have found that 4GB cards are a good size, but last time bought 8GB cards since they were a better deal. On the Pentax K20D with its 14MP sensor, 8 GB stores about 600 RAW photos. Day-to-day that is tons, but on a special assignment or trip I easily fill these cards.
You also need to consider whether you shoot video. On the Pentax K-7 an 8GB card can store 24 minutes of 720p video, which is plenty.
People favour different brands, but SanDisk always seem to come up on top. Never buy generic memory! Never buy from untrusted eBay sources, since you might very well get a fake or bootlegged card. If the price of a card seems to good to be true, maybe it is.
Look out for popular names that nonetheless produce inferior cards; Kingston and A-Data often fail reliability tests. OCZ and Crucial are reliable makes (known to system builders) if you do not mind slower cards, but I would not recommend them for larger SDHC cards. And despite all I have said, I have found good deals on "unbranded" Samsung cards, for applications where performance was not the main issue.
(In passing I note that the SD spec only governs cards of up to 2GB capacity. Larger cards are SDHC. But here I generally use SD to mean both.)
But to keep things simply and avoid disappointment, stick to SanDisk, Lexar, Transcend and Patriot.
If you have the cash you cannot go wrong with SanDisk 8GB Extreme III Class 10, a 133x card. From the ratings this should guarantee minimum 20/10 rates, but in fact has been measured at 27/25. This only goes to show how useless the ratings are! The last time I checked I could get one for €45, which was too steep for me. But deals abound, especially in North America, where one can sometimes score a great deal on an instant in-store rebate.
Instead of the Sandisk, I purchased the Transcend 8GB Class 10 20MB/s which tests at 20/17 and cost me exactly half as much per unit. So far they have worked out very well. I think the speed is more than enough and reliability (according to others) is top-notch.
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