Is a "high speed" memory card necessary?

More specifically, for the sony memory sticks PRO duo cards. In what seems to be classic Sony style, they've come out with a multitude of different memory stick options, some big, some small, some you need an adapter for, some are high speed, some aren't. They're all over the place, but to get to the point; Is it necessary to have a high speed card, if you own one of their high end digital cameras and expect to take lots of video? Does it really make that big a difference?

Thanks, and sorry for the ramblings.....