Well Nick, i think most of the gear heads around here would say you've already thrown some good money away. The headers, intake and exhaust will get you a few more HP on the top end at the expense of some torque on the bottom end. The Jet chip will fool the computer into running rich and thats about it. (you can achieve the same with a 15 cent resistor in the right spot I have heard)

If you're running big tires and have lost some grunt, you need to re-gear the axles to a lower gear ratio.

If you really want hp, Avenger and Hesco make a supercharger. The 4.0 is solid and reliable and with proper maintenance should go over 250,000 miles, but it was designed for low revving offroad situations. All that crap in the magazines will empty your pocketbook real quick in exchange for 10 hp at 5,000 rpm, and how much time do you spend at the redline?

see also Big Jim's replies to similar questions!

Josh