I don't mean to flame ya or nothing, but you've been asking a tons of questions in one day! I'd really advise you take a look around here and search the current forums for some of the info you seek. And then get a plan together. What do you want to do? Are you building a 1/4 mile burner, a daily driver, an auto crosser, or street monster or what? Think about that and think about what it will take to achieve that goal. What is important to you? Speed, ride, stereo, comfort, acceleration, handling, looks? All the above, right? But what do you want most that fits your budget? Or that you can save for that's not a complete dream?
But for most apps, you're headed in the right direction - headers, midpipe and catback work great on the SN95. And to answer your question, long tube headers (best for power but most expensive), a O/R midpipe and a free flowing catback will likely net you about 20 RWHP. Some more, some less.
Again, I'm not trying to discourage, just trying to make sure you don't aggravate too many people with too many questions - And I think you'll be happier if you have some sort of direction to go, rather than buying this, that and the other only to find out they don't really work, don't work well together, doesn't get you this or that - whatever.
BTW, did I hear you say you installed the 3.73s yourself? I'm impressed!
Best of Luck and Welcome to StangNet!
I got a clip of it... I can email you or if you have AIM, I can send it through there. But long tube headers tend to smooth out the tone (not as much cracking popping sound) So some might say it's louder and some might say it's quieter. All I know is when I rev it up a bit.. people will turn head