This site has a test report on card speeds.
http://www.robgalbraith.com/bins/con...id=7-7896-8483
For me, a couple of advantages of faster cards are speed in emptying the buffer, (raw + full res Jpeg fills the buffer quickly), and also copying card contents to my portable storage device. IE the faster I can transfer the data to the PSD the longer the battery lasts.
If you have ever tried using an IPod as a PSD you will know what I am on about. BTW I have a Nexto PSD with a 60GB drive which works very well.