You can use a circular in place of dpns, but for a hat, there's a coupla tricks to know. Usually you would use a 16" needle for a hat, but at the top after it decreases you won't have enough sts to go around it easily. So you can use a longer one for the whole thing with the magic loop method (see the advanced techniques page) and that works for the top too. Usually you'd want about a 36" long needle, but you can use a shorter one and the single loop version
of magic loop, at least until you get down to about 30 sts and you'll have to ML that too.