Go get whatever muffler you want...Magnaflow is nice on V8's by the way...and go to a local muffler shop, not Meineke or other well know places, but a locally owned place. Get single in dual out and call it a day! Get them to run the pipes and get your own tips or get some tips online or whatever you prefer. This will be cheaper and will be catback as well. I don't know if anyone has dual out on an '07 yet. This will be cheaper to do as well than buying a kit. Even that single in singl out kit Troy posted costs more than if you were to get dual. It should cost somewhere in the ballpark of $300 with tips, muffler of your choice (single in/dual out...size??), pipes, labor...?tax?... Good luck!
-DD ST