if you want loud sound... do a true dual super 40. You're not gonna get a performance boost, but it will sound amazing... i have my setup for sale. The pipes are already pre bent and just need to be welded on. It has dual tips that are 12" and chromed out. They exit out the passenger side behind the tire.
Check the For Sale area of titantalk and youll see some pics
If you want serious noise with some good power, than instead of buying another exhaust system, throw some Stillen or nismo longtube headers on. This will give you more noise for sure and a good kick in the power and torque department.