"We need to talk."
"Porozmawiać" is a perfective verb so it puts accent on the effect of conversation.
"Rozmawiać" is an imperfective verb so if you used it here, it would sound as if the act of talking were more important than the effect itself.
"Mówić" means "to speak, to talk" but not "to converse".
There is also a verb "pomówić" which is a synonym for "porozmawiać".