dodgepower,
Billysgoat is right turn out stacks will be the loudest you can go!!! Check out the MBRP stacks with turn outs ot miter cuts. They are the only folks making a stack kit with 304 polished stack pipes, no worries about chips and the chrome rusting that way!!
Another option is the 5 inch turbo back systemn with a 6 inch tip, not quite as loud as a stack kit but less expensive and close on sound.
As far as MPG goes getting more air thru the engine always increases the BTU effieicency of the fuel you are burning. Remember though that you'll have to keep the right foot off the go pedal and getting an new intake and exhaust that will be difficult for a couple of tanks. The AFE Pro Guard 7 is the best intake as far as flow and stopping dirt!!
If you have any questions let us know, we'll be glad to help you on them.
Mark @ DPPI