Class 6 write at more than 8MegaBYTES per sec. 
The best HD channels broadcast at around 20MegaBITS per sec. And the top AVHCD SD card cameras currently record at around 15MegaBITS per sec. So even allowing for protocol overheads, a Class 6 or Class 4, or, pushing it, even a Class 2 can cope with the current generation of HD camcorders.
Class 6 write at more than 8MegaBYTES per sec. 
The best HD channels broadcast at around 20MegaBITS per sec. And the top AVHCD SD card cameras currently record at around 15MegaBITS per sec. So even allowing for protocol overheads, a Class 6 or Class 4, or, pushing it, even a Class 2 can cope with the current generation of HD camcorders.