I just, as of yesterday, had a dual exhaust installed on my 2004 3.9L V-6, and it sounds great. Maybe not loud enough for some, but I think it's going to be fine for me. Through help from some of the members here, I decided on purchasing a "take-off GT exhaust", which is a complete exhaust system removed from a GT, and that's what got installed. I bought mine off eBay, removed brand new off a 2004 car, and it cost me $140 + shipping. I was going to put FlowMaster Delta 40 mufflers on as well, but after a mishap with the mufflers and with the muffler shop, the stock GT exhaust was installed instead, but it sounds pretty damn good if you ask me. I did listen to a lot of sound clips off the previously mentioned website as well, and found it helpful in picking out a "sound", as it seems each of the different brand mufflers sound different.
Good luck!
-Lynn.