Let me help by detailing a few mods I have made to my 99 V6. I went with the California Mustang dual exhaust set up. I think the price is now $220, but it comes with an almost new (20 miles or less) exhaust system taken off a GT and the H-pipe. THe H-pipe is required for max. HP when converting to a dual setup.
To answer your question about H-pipes and cat-backs, an H-pipe replaces the Y-pipe in your exhaust system. If you look under your pony, you will see the cats followed by a Y-pipe that takes the dual pipes from the cats and merges it into one pipe that then flows into 1 mufler and tail pipe. When an H-pipe is installed, the Y-pipe is cut off just after the cats. The H-pipe is welded into its place behind the cats and then flows into dual pipes and dual mufflers and tail pipes. As far as a cat-back system is concerned, it refers to any exhaust system that starts behind the cats. It is a violation of federal law to remove, modify, or replace a cat that is functioning normally. They can only be replaced if they fail. So, if you are to adhere to the law, you can only install a cat back system unless your cats are damaged.
I like the dual kit I installed. The car has more power and revs faster. It is worth the money.
As far as the cold-air kit goes, don't waste your money. All you are getting is a chrome tube with a K&N filter. CAI kits give no HP gain over the K&N filter. So, either save the money and get your own K&N filter or do what I did -- get a whole new mass-air kit. The Mac and other CAI kits use the stock mass air sensor (mas). The stock mas is the most restrictive part of the intake. So you get less restricion from the K&N filter, but the stock mas still restricts how much more air can flow. Look into the C&L Performance Mass Air kit. They reuse the stock mass air sensor electronics, but you get a new mass air sensor housing that is milled aluminum. They can also manipulate the calibration to give you a more rich or lean mixture. Instead of getting a custom chip, they could give you a recalibrated mass air tube to compensate for the altitude without you having to buy a mass air kit and a chip. Let them know your situation and they can advise on your best options. I have the C&L kit and swear by it. A CAI kit gives you 2-3 HP over stock (ironically the same HP gain as just a K&N filter over stock. Coincidence???) as opposed to a 6-9 HP gain with the C&L kit. Check it out before you make a decision on the CAI kit. It may not look as pretty, but it does work.
Here are the web addresses for the companies I mentioned:
California Mustang:
www.cal-mustang.com
C&L Performance:
www.cnlperformance.com
Hope that helps. E-mail me at
russman17@netzero.net if you want more detailed info. It is very easy to destroy a car by making bad modifications, so I thoroughly research every thing I do to my car. So, I can provide a lot of good advise if you are interested.