My primary cards in my D300 are Sandisk Extreme IV and Extreme Ducati 45MB/sec. cards. I have tried the Sandisk Extreme 60MB/sec. cards in my D300 and they are slightly faster but nothing to write home about. That said, the maximum write speed is limited by the camera at about 30MB/sec. or so if I remember correctly.
Barry, I have been using Sandisk 8gb Extreme IV cards since I bought my D300 new. As Marty points out you won't get much benefit from faster cards in camera. But where you will benefit with faster cards like 60 mb/s cards is when transferring files to your computer using a card reader and not your camera.
As has been stated, you will not see a lot of improvement to writing speed from the camera with anything above a 45MB/s card. The actual capability of this body is about 30MB/s so the only improvement you will see is in the read speed to computer. A fast card will transfer your images in less time - obviously - but only you can decide if this is really important to you. The cost of fast cards has come down recently so it becomes less of an issue, but the faster the card - the more it costs and if there is no benefit it could be considered as money wasted. Having said that, some have suggested that your next camera might take advantage of the extra speed so the card becomes an investment for the future. RAW files from the D300s are about 16Mb so a 8Gb card will take about 400 images in 12 bit lossless compressed format.