Bolt-ons
K&N CAI
Got it on sale at PFYC.com, took me about an hour to install. The whistle at low rpms are great, but the sound you get when you put the pedal to the floor is wicked!
SLP PowerFlo
New vs. Old
The y-pipe was bent a little odd, so it made getting the right side some clearance from the bumper a pain. Right now it's very slowly eating away at the bumper.
Here's a before/after video of the exhaust system:
SLP PowerFlo on 00' Grand Prix GT - YouTube