If i install a new exhaust system what will it do for my gas milage numbers? Also is it much louder than the stock? Also what is the hp gain with it? If i do end up doin this which one should i get.

I have a 2004 Vibe and was wondering how putting a ram air or cold air intake on the car would affect the gas milage. Also if I do get a ram air or cold air intake which one should I get?