increasing the bore of your engine will give you the best torque results.
besides that, Turbo will help somewhat. (both are very expensive)
almost all bolt-ons (exhaust, CAI
, shortram, header) will decrease your initial torque, but increase your top-end torque by a few whp). so on the cheaper side a header would do best, then exhaust, then CAI
or shortram. but again..it will only be noticeable on the top-end. at low rpm's your torque will actually decrease.