In a relatively simple series of tests done by The Sports Photo Guy it is pretty clear that going to higher speed cards does provide some write speed increase. A look at the test results at the link below shows that the best cards around 300x allowed write speeds around 23 to 25 MB/sec, while moving up to good cards around 600x resulted in write speeds of 28 to 31 MB/sec. So... doubling the card speed rating provided a marginal increase in camera write speed. Here is a link to his D300 test results: http://sportsphotoguy.com/d300-raw-burst-test/ A couple of notes: 1. This series of tests was with raw (NEF) files. I think he may have another test on his site with JPEG files. 2. Some cards performed very poorly that claimed relatively fast speeds - see the bottom of the list.
In a related issue, *IF* you have a fast card reader with a high speed connection, going to higher speed cards can make a big difference in the speed that the files are uploaded to your computer. In addition, a faster card than your camera can actually use today may be just right for your next camera.
My approach is to buy the fastest card(s) that I can comfortably afford in order to get good performance from my camera and reasonably fast upload speeds to my computer. The cutting-edge, really fast cards are too expensive for me, but one or two steps down from there fits my needs perfectly.