
Cams!!!!! FR2's is my recommendation along with a SCT X3 from Turbo Tom (I don't care if you like him or not, tunes for life can not be beat).
CAI - maybe 3 WHP and costs roughly $300. Not worth it IMO being the SVT stock setup has proven to flow just as well and perform as good.
Throttle Body - 0 WHP and costs roughly $300. Basically a little better throttle responce, whoop-to-do.
Honestly most of the bolt on stuff for the zetec is a load of shit. I have/had every bolt on out there at one point or another. I can say with extreme confidence that most are a big waste of money and make you sound like you know what your talking about but only to someone who knows nothing. Most of it is just trying to justify why your now broke and still can't beat the fuckin' Civic

Cams and a tune are the second best bang for the buck, NO2 being #1 (but depending on how you factor recursive costs it can fall pretty quick).