Welcome. Rob Galbraith's site (http://www.robgalbraith.com/bins/index.asp) is probably the best reference for camera/card write speed info. However, the chart for the D300(s) is no longer up. If I remember, however, the camera's fastest write to card speed was 30MB/s, with the fastest available card at the time he did the testing. Any advantage gained by using the faster card would be downloading to the computer with a card reader.
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.
The above sportsphotoguy link is for the D300, the D300S is slightly faster. The 600X cards seem to be about 15% faster than the 300X cards in the S body. For the non-S body, I think the difference is about 5%.
Again, 600X on one brand might be half the speed of 600X on another brand while a third brand might be significantly faster than their rating. I've found Kingston CF cards to be completely reliable but rarely even half their rated speed. Their 266X card is slower than their 166X card, for example.
I have a couple of 600x cards which certainly read to the computer much faster than the 200x ones. In normal use there is no real advantage since, unless you are regularly taking bursts of shots, the buffer easily clears before the next shot. I would go with the idea that it could prove "future proofing" if some new body that you finally afford gives higher write speeds than the D300s ! That said, personally I don't think I will be buying for quite a while, I gave up "chasing the cutting edge" years ago.