I see people buying the fastest cards at times. Keep in mind that the difference in fast burst rates is very slight from moderate speed cards to very fast cards. The difference really only shows up in how fast you are ready to do a full burst again.
Unless you do bursts of more than 17 images, or several bursts in a row of about that many, you will never fill the buffer even at full (compressed lossless) frames. And all but the slowest cards do video fine.
The only place I have found it matters is, when doing primary + backup (which is more or less the sum of the time for each), it takes a while before you can use the preview and controls to review and delete shots. I've found the 40-60mbs cards perfectly adequate even in primary+backup, and find it far more convenient to put the added money into larger cards.