just buying a muffler won't give you any hp gain I wouldn't think
all it'd do is just...give it a different sound, muffle it yeah? =p

I once heard, the best way to help your body function is to help it breathe
help your car breathe and it functions
I bet a K&N intake, maybe a new intake manifold, would help
then, if you wanna do your exhaust right, I would go catback
and if all you want is looks, buy a muffler tip and slap that badboy on, but don't expect any sort of gain, it's all aesthetic

Or you could try and clean up the auto a little, make it shiny, sell it/trade it for a MT and then start your upgrades and modding

 

k&amp!!