exhaust questions

where is the most performance and/or sound gains found?? at the headers? at the midpipe?? in the catback?? this is my concern because i want to keep the expenses under 1g for now. a friend and i will be installing all exhaust components as long as they are bolt on so i figure i can afford two parts of the exhaust system for now. so what are the suggestions... should i get headers and a midpipe? headers and catback? midpipe and catback? the 3rd component i plan on acquiring at the end of the summer so i just would like to get the most gain until then.

so far im thinking bassani headers and xpipe, and magnaflow catback..

also, what kind of effect do cats have on performance/sound/emissions?