There are two basic ways you can go. One, you can buy a dual exhaust that bolts up to your y-pipe. This will give you looks, and better sound than stock (much better). MAC and
Flowmaster make those and they might cost you $300-400 installed. Two, if you do have some extra money you can make it a real dual exhaust. You can add some JBA headers around $450 (optional) , X/H pipe $300-400, and a dual catback exhaust $300-400. This will give you much better sound, and better performance, but is probably going to cost more than $1,000.
Good luck with whatever you choose.