I've seen a lot of deals lately on Class 4 and Class 6 SD cards mixed in with deals on Class 10 SD cards.  The price difference between these classes of SD cards is now nominal and, as a result, there's really  no reason you should buy a Class 4 SD card anymore.

The prevalence of this problem came to light recently when a friend brought his Canon 60D to me, stating that it wouldn't record video for more than a couple of seconds.  My gut told me to check the SD card first thing.  And, sure enough, he had an old SanDisk Ultra Class 4 SDHC card in it.  I swapped it out with one of my SanDisk Class 10 SDHC cards and it worked like a charm.

He ordered a new Class 10 SDHC card that day.  His 60D has captured video just fine ever since.

If you don't already know, the Class-ratings on SD cards relate to a minimum write speed. When recording HD video, you need a minimum-sustained write speed in order to dump the data off the camera and onto the card.  In many cameras today, Class 4 (4MB/s) and Class 6 (6MB/s) cards just can't handle the data transfers.

Class 10 cards are currently the fastest rated for minimum write speeds available and will handle video capture for most, if not all, current HD video DSLRs and camcorders today.

If you want to know more about SD card speed ratings and classifications, check out my article on Demystifying SD Cards.

If you want an easy recommendation, go with SanDisk Extreme Class 10 SD cards.