what are you using it for?
gaming? go with the 550, higher clock and L3 cache make it a better performer in most games.
but for other tasks, 4 cores is better. so go with the 620.
as warmon said, your mobo needs to be compatible. but i dint think there are many, if any, mobos that accept phenom's but not phenom IIs
Well there are some motherboard the accept phenom I but no II's. What i was looking into was making sure that the cpu the op was looking into would work. I know some board that have accepted some Phenom IIs but not all. If the op wanted a phenom II x4 955, he cant with his MB. To OP
Yes both cpu's your looking into will work on your motherboard.
As welsh said, depending on what your doing go with what cpu best fits your need. (just be sure to update bios before the changing the cpu!)http://www.asrock.com/mb/overview.asp?Model=N68-S