first of all you need to know what hp gains you want and then there are different ways to get there and things to be concerned about---
here's two of the main concerns when you upgrade the HP:
auto tranny or clutch
EGT's(Exhaust gas temperature)
since I have the 24 valve pumping out a dyno'd 397 hp(on diesel) I can tell ya little here--
there are different boxes you can add that can up the HP to different amounts--most of them are adjustable with the push of a button or the turn of a switch--adds 100hp +/-
new injectors that can add anywhere from 40-60hp, 80hp, 100hp, 125hp, 150hp & 200hp---now remember these are the advertised hp ratings some vendors are closer to their claims than others---
4" or 5" exhaust 7-15hp is the advertised, not to sure this is true--but they do sound cool
bigger turbo---lets you move more air to use those injectors to burn the fuel
propane--up to 100hp+/----there are different gains using different jets
NOS(nitrous)---up to 100hp +/---again due to the nozzle size
changing the fuel delivery system could be beneficial also----
if you're interested email me and I'll give you what info I've gathered----chris