There are only a few options when it comes to replacing your downpipe or cat.  Some people simply gut out there main cat.  But if you live in a emissions controlled area, you will be sunk next time you go in for your emissions test.  ATR and Random Technologies make hi-flow cat replacements.  ATR and Alamo both make downpipe replacements.  ATR also makes a test pipe that you can interchange with your cat.  Stillen also makes a downpipe that replaces the front pre-cat, downpipe and cat.

Alamo Motorsports downpipe