I just purchased a 2007 Nissan Altima 2.5s, and i was wondering how i could add horsepower and torque the easiest and cheapest and most efficient ways. Can anyone help me out?

I have a 2005 2.5s and looking to do a few mods to gain maybe 30-35hp or so without making the thing too loud. Anyone have any suggestions? What I'm thinking that would give me about 25hp... - Injen Cold Air Intake - Stillen QR-Pro Fuel/Air Management System - Underdrive Pulley (Anyone have any...