+1 above. if you really want to throw the car around on a racetrack, i believe you should upgrade your tires, brakes and suspension, in that order. depending on what track you regularly go out on, you'll then have a better idea as to what components to upgrade.
tracking a car is expensive, both in terms of entry and travel costs and also running costs of the car, the car will munch through its tires, brake pads, rotors and clutch and the oil will turn into sludge much faster than on the road.
gear, such as a helmet, a pair of gloves, measurement stuff (such as a box with high quality GPS and accelerometers) will help you improve your driving style and are transferable to any car.