in my opinion exhaust up grades are probably the best bang for your buck upgrade,some things to consider when designing it might include,primary tube size and length,tri-y design(where cylinders are paired)collector length and size,exhaust pipe diameter and length.
usaully bigger is for higher rpm but there are some compromising with a complete system.
alot of shops will just bolt on a set of crap headers and monster pipe and send you on your way,research and it'll all be worth it.
personally tri-y headers and 2 1\2 pipe worked for me I also have an x pipe and a scavenger set up it works well for me.