I was already getting ready to type...> use the search feature but by your name I hope your female cause im going to be nice enough to answer your question. = )
If you want performance gains I would definitely start the upgrade from the headers, all the way back to your muffler.
I've heard if you get an aftermarket header and keep the stock cat and muffler you will eventually blow up the stock cat.
If you just get a catback system and keep the original header I believe you wont get the full potential of the system.
If I were in your shoes I would go and buy a header, and a magnaflow cat back exhaust. ( Has a stock look to it but not stock performance ).
Also if you have the 4cyl version with the single muffler, stick to the single muffler as the dual will make u lose a bit of back pressure.
btw... never heard of powerflow......maybe magnaflows affiliate?