first of all, adjust your valve play/clearance. If you have a car with higher mileage, the wrong valve play may rob you hp.
then a chip (f.e. Dinan) is good for increasing hp low cost.
I'd add an exhaust system and a CAI (cold air intake) first, but do it however you like
have your car dynoed (especially after the mods) and then have it tuned by a good mechanic.
it all depends on your money...
you could also do an engine switch to an Alpina 3.5 engine or an Alpina V12
but I wouldn't do it, anything above a 6 cylinder puts too much power on your front end
also always keep in mind to improve brakes and suspension alongside with power mods.
BTW, welcome to the forum