to answer your other question:
it doesnt matter what you get first--exhaust or header.
usually an exhaust comes first because it is more noticeable (and cheaper for a gts). exhausts are both an appearance mod AND a Performance mod, which makes it more desirable. whereas a header is stricly Performance.
i'll put it another way: almost every user who owns a header has an exhaust, whereas most users who have an exhaust do not have a header (in an aftermarket Performance sense)