Positive displacement is definitely the right ticket. The Roushcharger isn't a bad route to take, if it's installed by an authorized installer, you'll keep your factory warranty also. However if you're not too worried about the warranty, look at the Kenne Belle lineup. Their results can't be argued with.
"Posterity! You will never know how much it cost the present Generation to preserve your Freedom! I hope you will make good use of it. If you do not, I shall repent in Heaven, that I ever took half the Pains to preserve it."