I've got the K&N 77 CAI and a Magnaflow muffler - my signature has the specifics - and I'm very happy. I think I paid about $280 at a local shop here in COS for the CAI alone. Since I put both on at the same time as the lift and tires, I can't say it increased gas mileage - but it definitely sounds and feels better! Here is a clip of how it sounds:
And you don't need to pay anyone to install it - very quick, straightforward and easy to do yourself. I think it took me right at an hour in the garage.
2006 Inferno Red 3.7L 4x4 QTI, RR 2.25" Lift, AEV Pintler 17x8.5" Wheels, 255/75-17 BFG M/T KMs, K&N CAI 77-1545KP, MagnaFlow Muffler, 4XGuard Side Guards, Rear Grab Handles, Front Tow Hooks and OEM Trailer Hitch, Fog Lights, Yakima MegaWarrior w/ Extension and 4XGuard Belly Guard.