the only affordable solutions to help increase horspower/torque (less than $1000) are like you said headers, Intake
, exhaust. also, as others have mentioned nitrous, pulleys, & grounding kits.
nitrous is the best horsepower-per-dollar option. although you have to keep buying it (although to refille the bottle it's not very expensive), it can provide as much power as a Turbo/supercharger (or more if you re-enforce your engine) for a fractoin of the cost.