No, you don't need ATX, it's just handy to have the room. Some mATX boards and ATX boards have the same features - but not many. Makers can put more options on a ATX, and, airflow is better when OCing.
My budget builds are OCd stable so I can get the most bang/buck.
If you are asking an opinion on the board choice; I would definitely get the ATX GA-880GA-UD3H. The SATA 3 and RAID options future-proof it better and it has USB 3.0. $12 well spent if you ask me.