It depends.
If the camera has a 5 shot buffer, and you take your first shot, the speed of the card does not influence when you can take the next one (or next 4, actually). You can do so without waiting for the camera to finish writing.
The speed of the card only matters when:
1) You want to preview the shot immediately after shooting and the camera only allows it after it writes to the card;
2) You hit the buffer limit.
3) Reviewing images that are not on buffer. Some dSLRs store thumbnails in the buffer, while some read from the card.
If your style is to shoot intermittently (ie w/o ever hitting the buffer) and don't bother to review, you'd never notice the speed of the card.
