speed of 90 MB/s. So I don't understand what the "speed >ratings" (400x; 600x; 800x, etc.) mean, or if they mean >anything. It appears that that number may be misleading. That >is one reason I started this post...to share information and >learn more about the ratings and relative values of these
As I understand it (and not sure if I do), the "x" it refers to was originally .15mbs. So a 400x card is 60mbs.
However, the rules for this seem a bit loose, e.g. read vs. write (which do you quote), and of course peak speed is only achieved with certain camera hardware.
When I said 20-30 was the range from slow to fast, I was quoting Rob Goldbraith, and his how many shots in 30 seconds.
I think the more interesting number is how many full speed shots you get, and there is almost NO difference there. Try it. Put the slowest card in and you'll probably get about 13-15 lossless compressed 14 bit. With the 1000x card you will get about 17 before it pauses and slows. In my experience it's just about a second of shooting difference between $30 and $300 cards for full speed bursts.
Now the time to finish dumping the buffer is highly affected by the card speed, so if you want to affect how long until the green light goes off it's very important. Which impacts subsequent shots, and total shots over a period of time. I am not pretending it does not matter, just depends on what matters to you.
But try it -- the D800's ability to burst at full speed is not very affected by card speed. You are largely limited to the buffer, plus up to about 4.