Lately, I had a lot of mileage with SD cards at work.
1) The speed the manufacturers print is not always the speed of the card!
2) Depending on how you format your card, you may also degrade its speed.
3) The faster cards may be overkill for the work you do with them.
I used a free tool used to benchmark SD cards called iozone (http://www.iozone.org/)
that is well known for "testing performance of hard disks".The SD cards were checked in different platforms. (Embedded Linux, PC with Ubuntu Linux, PC with Windows7).
Usually the write speed is limited by the capabilities of the device, in our case, the D7000. Using cards faster than the camera will not give you any advantage, therefore you can save money by getting slower cards. (The only advantage is that you will be able to download the photos faster using a faster card reader).
The speed of the card depends a lot on how you "Format" the card,
Formatting the card with the camera(D7000 in my case), and also under Linux(Using the default parameters for FAT32) caused some degradation on the card speed. The reason is that data access is not aligned with the hardware requirements on the card.
Formatting the card under Windows7 produce good results, similar to the ones using a card that was formatted by the SD Card manufacturer (the approved ones).
If the card comes already formatted, it will produce the best results for you, you may not need to re-format it, due to the fact that the manufacturers will format the card in such way that will produce the best results for the card.
My purpose was to share my findings, although I can not publish here the real results due to some "Legal reasons".