Hi, and welcome.
I would suggest some bolt on things, your really not going to add a whole lot of HP with bolt on's but some is better than none. Start with a cold air intake kit, you can get any where from 5-25 hp. Is it supercharged? If so go with a bigger pulley, that will help. If not go with some underdrive pulley's that can also get you any where from 5-25 hp, i think you can also get some alternator pullies that will give you up to 5 hp. Exhaust is also another thing you can always upgrade from stock. Not only will it give you a deeper and meaner sound, it also adds some hp. And quite possibly the best thing could be a chip or tuner. I havent experimented with one yet but they can give you a ton of hp over the stock computer. Depending on how in depth you want to go you can upgrade your fuel injectors, im not sure if you will get any hp gain but i think you may save a few bucks on gas. Like i said, im not quite sure on that. With less than a $1000 bucks you can bolt on and plug in well over 400+ hp.
i would start with the intake, its pretty cheap. check with americanmuscle.com or other websites for details. BBK, and K&N are 2 good companies i trust.