classes have to do with the speed that the card can write
its a real easy rule to remember

Class 2: minimum sustained DTS of 2MB/sec
Class 4: minimum sustained DTS of 4MB/sec
Class 6: minimum sustained DTS of 6MB/sec

(ps DTS=data transfer speed)

Well, as much as this is a personal preference sort of question..
I have mine filled to the brim with mp3s and run canola on it, its responsive enough to not notice a difference between different smaller cards
however, it is noticeable in the file manager and other full listing that doesn't depend on a database



Technically the SD Card association have defined the maximum limit of SDHC capacity to 32 GB, but thatll change of course