For the cheapest noise increase, try a resonectomy, where you remove the resonator from the air intake box. That will give it a bit of grunt for a N/A engine.A search of the forum should list a bunch of people who have done so already and instructions if you need it.
And if you want more, consider new mufflers as Tek suggested. If you want something brand name like HKS, contact Phat GT at
forumparts@gmail.com. Otherwise the best value would be to drop by your local muffler shop and get a custom set made up for you. The problem with using universal mufflers is it will be hit and miss when it comes to what noise you end up with. Where are you located? Perhaps you can ask to listen to a custom setup of some of the members on here who have done it if they're local to get an idea.
And last but not least, you can try this solution here:
viewtopic.php?f=5&t=8396&p=119933Get that race car sound .... through your speakers!!! Just have to leave your windows down. I wanted a group buy on this but was promptly shot down.