fans would be your next best purchase for larger HP, torque, and MPG gains.
by eliminating the clutch fan hanging off the front of your engine, it removes all the drag and extra power it took to turn that fan. which means all that extra power and torque is now at the disposal of your right foot. As long as you drive like normal you will also see an improvement of about 2 MPG immediately.
under drive pulleys will help even more by eliminating the rest of the parasitic drag from the serpentine belt, it does this by changing the size of three (3) of the pulleys which frees up 10-15 or so HP, and some torque, and some (like .2 -.5) MPG and does not change any of the characteristics of how the system runs or charges.
I think the kit changes the size of the crank, idler, and alternator.
also, don't worry about headers, unless you can find LONG TUBE headers.
shorty headers (for the price) deliver almost no change. They are terrible to put in also because of the aluminum heads. (not so much going in, but it's breaking the bolts off while getting the OEM out.)
long tube headers (so I've heard) offer a scavenging effect which increases the H.P. and torque.