Back to basics...
Before getting nitrous, go ahead and get a open filter intake. Most popular is K&N + MAF Adaptor. After you do that, go ahead and buy some headers from Marty @ MSDS. Then decide which Y-pipe you want to upgrade to, and get a cat-back system. After that, you can either get a tune, or wait until you have the nitrous and get it tuned. Those upgrades won't net you 50HP, but they are basic modifications to set you on the right track. Other than that, look into a 3L swap, which isn't switching heads, its switching the entire engine block plus components (depending on which swap you do). For details on where to buy all the forementioned parts, or to do a little homework and learn more about the car, do a search on the items. "headers" "cat-back" "intake" etc wil give you enough reading material. If you run out, check out the old forums and search the same terms and you'll be reading non-stop for a couple years. There is a lot of information here, take your time and learn some of it so you don't have to be told what you can do, you'll already know.