Exhaust system, flowmasters?

Hey guys i just joined this forum and registered. you guys seem to know a bit about mazdas and well i'm bought myself one back in 03. it is a B-3000 V6 engine, Dual sport model. Anywayz i was wanting to put an exhaust system on my truck to make it sound a bit tougher and possible add some power and gas mileage to it. I do not know which way to go with this or how to do this at all. I"m hoping that maybe one of you guys might have an idea as to how i should do this. Should i use a Flowmaster or some other name brand exhaust. I did want true Dual exhaust (piping comes straight out of headers) but its kinda expensive and probably wont do me any good. i was thinking about switching the catalytic converter on my truck since some people tell me that factory catalyst are not efficient, then one of my buddys told me that i could take it off and put an after market catalyst in but that if my truck was "backpower dependent" then i would actually reduce performance on it instead of increasing. I don't know what that is but if you guys have any info then tell me please.