Buy a header and a high flow cat, take it to a muffler shop and they can modify your exhaust so the cat is after the header. make sure you get a true highflow cat and not a resinator. A lot of "performance shops" like to sell straight through resinators as cats. Those are not legal, and most (legit) shops will not install that inplace of a cat.
A highflow cat should run you about $60- $80 and a header will run $200+ depending on the quailty and style. Put the header on yourself (to save cash) and have the muffler shop do the cat, it shouldn't cost more then about $60 to have them put the cat on (welding included).