The SD card class/speed has little to do with write speed. The speed class is based on very large sequential writes using the SD write multiple command with pre-erase and the 4-bit SDIO bus.
The Arduino uses the SD single block write command with the SPI bus. Often very cheap class 2 cards have better write speed on Arduino than class 10 cards.
You need to benchmark the card to see what its performance will be on Arduino. You can use the SdFat bench example.
SdFat is the base library for SD.h and is here http://code.google.com/p/sdfatlib/downloads/list