I bought an 8 GB Sandisk class 4 for my wifes 620 and it performs excellently. You may want to go for a class 6 card for guaranteed performance but a class 10 is total overkill for a budget smartphone like the 620. I've also got a Sandisk class 4 card in my Samsung Galaxy S 3 and it records videos (and plays them back) without missing a beat.
Other people will tell you to get a class 10 but as I said they are not needed.