1) There are two different speed ratings that matter to still photos and video. When you're shooting stills, pay attention to the maximum transfer speed, expressed in megabytes per second. But for video, what counts is the Speed Class rating, listed as Class 2, 4, or 6.
This designates the minimum sustained write speed for video applications, whether you use a camcorder with flash-memory or the video function of a digital compact camera or DSLR. For standard definition, Class 2 is all you need, but if you're shooting high-def, spring for Class 4 or 6 (which cost $10–$15 more).