Header, Intake, & Exhaust - Mods to 2005 Civic SI

If I were to add quality stainless or ceramic headers, high-flow cat, cat-back exhaust, and a cold air intake to a 2005 Civic SI, how much would it increase my horseposer?
And, am I better with stainless or ceramic headers?
And, do I really need the high-flow cat, or can I bypass it altogether?

I'd pass on the hi-flow cat and use that money for something else. Your not going to get much out of that until your running some decent horsepower or you're running out of ideas on what to buy. Header... I'd go with stainless steel (thats my preference). After its all opened up I'd guestimate about an extra... 15-20hp

thanks for the advise. I currently have a 2000 Civic DX and I've added the cold air intake, and Genie cat-back, and have been very pleased with the results. I've been eyeing the 2005 SI and seriously thinking about buying one and starting w/ the intake, headers and exhaust and then take it from there.............what would you do after that??

after that i'd look into upgrading your computer by buying a programmable ECU or "chipping" it. Or you can go the way of suspension...

do I really need the high-flow cat, or can I bypass it altogether?
If your cat is on the exhaust manifold itself, you'll need to replace it. If it's not, you can bypass it.