[blue00gt]

Hammer is right about longtubes. My car is lowered over 2" and I only have slightly over 3" of ground clearance under my cats because of the longtubes. I am very careful and try to take everything at the right angle. so I rarely scrape on driveways or speed bumps, but there have been a couple times when the cats scraped just driving along due to humps in the road that I couldn't avoid without hitting something else. If you are willing to put up with the ground clearance issue, they definitely add noticeable power - I have pulled away from a couple of stock 5 speed 99+ GT's without a problem since getting them.
If you decide against longtubes, I would just get an H- or X-pipe for now and skip the headers because shorties don't seem to gain much power at all on the newer cars (I've heard maybe 2-5 hp depending on what other mods the car has). In my opinion they are not worth the cost and difficulty of putting them in unless they are one of the last mods you are doing to try to get every last hp out of it.