Please do not listen to anyone that recommends Xforce, they are so full of fail you will wonder why you wasted your money on chinese crap.
3 parts: Extractors, decent hi flow cats, catback.
Extractors: Most people do pacemakers for a reason, they are tried and true. Tri Ys are better for street as they provide more response across the rev range (or so they say) whereas 4 into 1's will probably give you a better dyno figure.
Cats: Why spend all this money on an exhaust and leave your shitty old cats on? Buy some decent ones, look for 200cpi ones, you can get 100cpi ones but their effectiveness is questionable.
Catback: Twin 2.5 inch is usual if you plans dont extend out past an OTR and tune, otherwise go twin 3 inch for cam, stroker, etc. Manta is good, so is Pacemaker, Difillipo systems sound awesome, but you pay for the note. You can get it with full mufflers (2 centre ones, then usually 1 rear muffler), or you can get just the centre mufflers and a straight through tailpipe, depends on if you want a nice burble or if you want nanna to hear it 7 blocks away. You can just get a catback if funds dont allow extractors+ hi flow cats, if you want a decent power gain though and the best note available get the lot though.
Pretty much if you want I can PM you some stores ive gotten quotes from, that actually offer decent systems. If you go into a shop and they start going on about how good Xforce is just turn around and walk away.