What's a good cat back exhaust to get that is QUIET and will give me a subtle power gain?

 

For a mild N/A build, I'd recommend the Apex'i World Sport II or the RSR Ex*Mag II (if rust is a concern). I run the WS2 and it is superb. A nice, subdued note at cruising speed, no resonating issues and a wonderfully complex symphony at WOT.

For a turbocharged build, I'd wait and go custom once the turbo is on, or buy a straight through setup like the GReddy SP2 or Tanabe Racing Medalion.