best bang for you buck is a hard one with these trucks because there isn't much aftermarket support
try reading up on what people are doing to the vg30e motors just google something like "vg30e build"
intake and exhaust will get you maybe 5-10 ponies if done right
cams are going to run you 300-600 depending if you buy new or have your current ones reground but they are good for 30-40 horsepower at peak output
your can have the heads ported and polished but expect to pay 5-600 at minimum at a decent shop that knows VG motors for a rather basic port and polish
the best way to make power with a nissan vg motor (you have a vg33e) is to go forced induction
the VG blocks are incredibly overbuilt and the weakest link is usually the cast crank or pistons and there are guys with turbo VG motors in 300z's, s12's, s13's, 240's ect knocking on the door of 500 horses with stock motor internals. and they are doing it reliably
not to mention after you make serious motor changes your dues for an ecu upgrade, either swapping to a standalone system or going to an easily reprogramed ecu a shop is familiar with such as a z31 or z32 ecu
but the engine control stuff is something i've only started to learn about. It is very possible that the frontier ecu prom is just as easy to reprogram as the z31 stuff. but like i said I'm just starting to really delve into the details of ecu prom and all that jazz