I'm just a little confused now
 

Okay, now I know this is probably gonna sound like a dumb question. But can someone tell me about Y-Pipes, B-Pipes, and any other pipes I would need for a full exhaust? I know you got the header, and the cat, and the Y-Pipe and B-Pipe? It gets confusing there. Thanks.

Re: I'm just a little confused now
 

From front to back.. Exhaust manifolds(aka headers), Y(F)-pipe (also known as the downpipe), catalytic converter, B-pipe, muffler.

You would need everything from the Y-pipe back.

No one makes aftermarket headers for the Maxima. It was proven not to be worth the trouble some years back, if I'm correct.

Correction: one company makes headers. Still not worth the trouble. :)

check the FAQ

arospeed makes headers

Headers will yeild at most 1hp to the wheels.

Why is it that aftermarket headers don't yield any significant gains? Are the stock ones a good enough design?

Something about something. I emailed the guy at arospeed for info and/or dyno sheets showing actualy gains on their supposed headers for 95-99 maximas. Probably wont get a reply as to the fact that arospeed sucks and is run by a rice cooker.
If he emails back i expect a full list of excuses/lies about how good their products are and what we hear/read is not only baloney, but written by fools who know nothing about their cars.

I heard there's an OBX header for the 4th Gen Max. Anyone know if it's any good? And thanks for clearing that up. I'm definitely going to be slapping down an exhaust system this summer. But should I buy the system even if I don't get a header? Is the stock one free enough?

Dont get headers, they are a waste of money. ESPECIALLY from obx

Header pipes are useful on V8 or perhaps inline 6 cylinder engines where four or more cylinders are close together in a line. It makes for easier packaging and design.

Anyway, the main issue with the stock Y-pipe is the junction between the left and right banks is totally non-conducive to good flow characteristics. This issue is corrected by all aftermarket Y pipes.
But also all the aftermarket ones have the pre-cats removed.

I would estimate half the flow gains come from fixing the left-right junction and the other half comes from not having pre-cats.