HDSLRs accept either Secure Digital (SD) or CompactFlash (CF) cards. Some, like the Nikon D300s and Canon 1D Mark IV, can accept both. The question — and confusion — comes down to selecting which speed to use and whether it needs to be UDMA. HDSLRs only write about 5MB per second, so technically speaking, some of the slowest and cheapest cards on the market can be used. On the other hand, there have been problems reported with using slow and non-UDMA cards, namely:
Aside from these possible problems, there are other good reasons for getting faster cards. First, high-speed cards can transfer files more quickly to a computer or storage device. When dealing with 32GB of footage and only a few cards to swap, transfer speed becomes essential.
Second, faster cards are more future proof. HDSLRs of today are just the tip of what will be offered in the near future. Higher bitrate video recording is around the corner, and today's faster cards have a better chance of being compatible with the cameras of tomorrow.
Last, if plans call for using the camera for high-speed bursts in still mode, or the ability to shoot stills while recording, then Canon recommends UDMA cards. These cards should have a minimum rating of 233x, but 400x is optimal.
In terms of how much capacity is needed, there are two schools of thought. The first is to limit the risk of possible loss, damage or file corruption by affecting only a small amount of footage at a time. That means using small 8GB to 16GB cards, which hold about 24 and 48 minutes, respectively. The second is to have higher-capacity cards so that they don't need to be swapped out as often.