Depends on what you mean. True dual exhaust (single pipe coming from each header all the way out to the back) or a quasi-dual set-up (headers into a 2 to 1 pipe [y-pipe] to the back where it splits back into 2 pipes)
True dual answer: run 2" diameter pipes, anything else is overkill, unless MAYBE your running some extreme forced induction. But 2" is what you want.
Quasi-Dual: Go to 2.5" diameter piping. As mentioned previously, the stock svt exhaust is 2.25" and you'll want to go bigger than that. The cheapest cat-back systems out are from
www.Trubendz.com
If you choose to go true duals it will be a custom setup. And of course you can have resonators or not, mufflers or not, cats or not, balance pipe or not.
If you choose to get a cat-back kit, I suggest you also get a optimized y-pipe, or a trubendz y-pipe, or something more free flowing than the stock Y (the stock Y-pipe is pretty restrictive). Also a Hi-Flow cat would be nice as well.
As far as Trubendz systems go, some people have fitment issues and have a hard time getting the tips centered, while other have no issues. Personally, I had to heat up a hanger and bend it about an inch for perfect fitment. Since you are getting headers, I would suggest picking up a resonated cat-back from them with a magnaflow muffler. Non-resonated is about $100 cheaper, but with headers and trubendz exhaust, it is LOUD. So resonators will help tone it down.
Hope this helps...