Short answer: class 6 doesn't exist above 4 GB in microSD cards (it does in regular SD cards)
Long answer: class 6 doesn't exist above 4 GB in microSD cards (it does in regular SD cards), HOWEVER, there is exactly ONE game that does not work correctly with class 2 cards: Castlevania PoR.
I have a 16 GB class 2 card, and filling it up entirely took me 15 minutes.
also please note that the class means the WRITE speed, READ speed is really not related, for that only the company that makes them matter. PNY cards are the best when it comes to read speed with kingston a close second.