A high GPA never hurts. More important is to intern with the office and perform solidly. Get a few senior DAs on your side. DAs are trial attorneys so they care about court performance. Do the things you know they want to see - mock trial, crim clinics, take adv. crim pro, intern with the DA & AG. Then do the best you can in school.
These days, at least in CA, it's tricky to land an actual DA position in a big city, but not necessarily because of GPA requirements - there are just a lot of people that find the work interesting and want the job.