You'll be fine with a class 2 since aside from initial copies, you'll probably be doing more reading than writing to the SD card. Keep in mind that the SD card's class is the minimum write speed in MB/s. But, that's just minimum. When I robocopy'ed my 16 GB class 6 to my 32 GB class 2, I got over 4 MB/s average speed despite the card being a class 2. The class has nothing to do with read speeds, which are normally very high with all types of flash memory.