hulk: x pipe doesn't really change the sound all that much, it helps keep exhaust flow even thru the pipes. this keeps back pressure more constant and looks cooler on those cold mornings or if u get water in your pipes u won't see it reverberating back and forth. this is much more noticeable with straight pipes(no muffler or cats) or glass packs. true duals means 2 mufflers with pipes run from each manifold seperate. x-pipe is optional and isn't really that effective unless your running over 300 hp i have heard.
Mario: flowmaster mufflers have to be welded in i think. midas or meineke are really cheap on labor if it comes to simple cut out old one weld in new one.
as far as cats im not sure. i just know if u lose em it pops the SES light
update:
i just lost a bet. my friend said flowmaster makes 10 series and others. i didn't believe him but just found em on their website. i wonder what the 10 series sounds like. it says its a "race" model so i'm guessing it ain't street legal. not that i care about laws like that which violate our civil rights anyways. cough...window tint...cough