Best significant, inexpensive, CARB legal HP boost?

My friend has a 1999 Prelude SH which has the H22. What would be the best way to get a somewhat significant and inexpensive horsepower boost while keeping the car street legal and using CARB exempt parts? The only mod on the car right now are DC headers.

aem short ram intake, high-flow cat and or exhaust, just check that all parts are carb legal