SDHC Speed Class
If you shoot video with a DSLR or camcorder that uses SD memory cards, be sure to use a card that is fast enough. Since the introduction of SDHC, cards are labeled with a Speed Class that indicates the sustained data transfer speed. Class 2 cards deliver a sustained read/write speed of 2MB per second. Class 6 operate at 6MB/sec. It’s an easy standard to understand.
The usual recommendation for video use is Class 4, but buy and use faster cards if you can. Transcend recently introduced a Class 10 card. Panasonic has Class 10 cards, too, and so does SanDisk.