your just looking at mufflers? or axle backs or a cat-back system.
i have the borla cat back on my 03 lx right now and i cant complain. its pretty quiet but with a slight tone to it. no drone on the highway or anything. just road noise.
i had the mazdaspeed axleback upgrade. that sounded better to me, but it slightly drones on the highway. it isnt bad but to some people its annoying.
im not sure since your car is a 2000, if the exaust system is the same as the 01's and up. search around and youll find plenty of info on it.