OP, some good advice in this thread. If you have never autocrossed before, I would do as others have said and run what you have now, to determine what is needed next. Maybe you won't even like it(autocross), and now you have spent the money for something you don't even have the desire to do.
Having a nice set of tires, does make a big difference, and probably accounts for about 40% of the 'set-up' on the car.
The best modification is ALWAYS the 'nut behind the wheel' because I can't pretty much guarantee you that there are a handful of people that could probably jump in your car on any given autocross course and go faster than you by 2+seconds.
So, learn to drive the car first, MAYBE put a good set of tire on there, and then just drive the car and see how the car performs, and if you like doing it.....THEN look at modifying the car further.
My .02....hope that helps.