Elizabeth, your publisher's publicity department will likely be sufficient. Their job is to publicize the publisher's books. I know of several instances in which authors hired outside publicists to supplement the publisher's work, but it seems to me that the publisher would become less motivated to promote your book themselves when you've hired someone else to do it, you know? You could go either way, but unless your publisher's publicity department totally sucks (in which case, did you really pick the right publisher?), you don't need to.
Kristen