The best thing you can do for the money, hands down, is an improved intake. You can go with a k&n drop in filter, or a cold air or short ram intake, the best being a short ram intake with a cold air source. Another cheap, actually free, thing you can do is reduce the weight of the car. You can remove sound deadening, air conditioning, spare tire, ect. Every pound counts. The single best and cheapest handling upgrade are stickier tires, although you will have a trade off of increased tire wear. Switch to synthetic oil. That really sums up the inexpensive ways to noticibaly improve your car. From then on it starts getting more and more exspensive.