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.