As aforementioned there are a lot of factors and one factor also is "how restrictive is your stock system"? In the case of the Sentra GA16 engines, the basic bolt-ons (w/ local crush bent headers at that) have been proven on the dyno to give around 10 wheel hp (+/-)....

Research first and ask around for the best i/h/e set-up within your budget; cross-check with other owners with the same ride.