I played the performance engine game with two of my cars, first a 69 mustang with a 302 I overhauled, second a 96 Mustang GT. I put the most into the 96 4.6SOHC motor and it was really fun and faster than a stock C5 corvette and about equal to a C6. I spent over 6-7 thousand dollars under the hood of my 96 and while it was fun and fast and sounded like a monster it wasn't really worth it economically. Even with a car like that which people appreciate mods I wish I would have put my money towards a newer model cobra instead of my older model GT when I look at it...it just isn't worth it.
If I owned a car like yours I would steer very clear of aftermarket parts really because they will only turn away prospective buyers in the future. Think about it...you have a car that is suited for economy driving, not sporty driving. If you go and mod the car out you are only going to decrease the amount of lower RPM torque it makes and the crowd of buyers you will be targeting in the future are highschool aged kids that will not want to pay full price for it anyways. On top of making it harder to sell and taking away the practicality of the car you will only be looked down upon by guys who drive real performance cars which is pretty much everyone in the sport compact scene all the way through the muscle and exotic scenes...if it were my car and I was in your shoes I would spend the money on luxuries that can easily be removed upon sale of the vehicle...
Another note you should make is that there is a good chance the aftermarket header you end up with will be full length which will probably be considerably longer than your stock manifold and you will need further modification to the stock exhaust pipe to make it compatible with your new header...maybe somewhere in the 70-150 range (estimate) to have an exhaust shop do for you...
My .02 cents is spend the money on luxury and not performance or just put the money away...hell you could even throw the couple hundred bucks into a stock like Visa (V) to see if it does what mastercard did...if that happens you could have 500 bucks from your 200 in around a year...