With the fastest (90 Mb/s) cards you will be able to shoot in raw mode at 3 fps or so even after the in-camera buffer fills after 6-7 shots. These cards can almost keep up with the camera. With a 45 Mb/s card you will slow to around 1 fps after the buffer fills. Any slower and you are really reduced to single, spaced-out shots after the buffer fills. In JPEG mode, you can go much longer until the buffer fills, so you won't notice the difference so much except for the backlog write time to clear out the buffer.
The 45 Mb/s cards are the "sweet spot" for price and speed, giving you reasonable write speeds for about $1/Gb. For fast shooting, you pay a premium.