I recently put a K&N FPIK CAI on my 2002 Ford Explorer 4.0L.  It made it run very nice.  Then I put a Flowmaster 50 delta flow and it sounded horrible.  Way to loud inside the SUV.  I am now getting ready to replace this with a magnaflow cat back system #15718 $400 with shipping at advanceautoparts.com.
Hopefully it sounds decent or I will be driving a car with no exhaust :-).