the single biggest chassis difference between CART and F1 comes from the methoid that both use to make downforce.
The CART machines use the undertray shape to generate the majority of their downforce. The wings are used as tuning devices to enhance turn in or rear traction. This venturi effect allows the engineers a much larger window of opputunity in finding a good setup and allows softer ride than F1. This also allows the CART machine to operate with a larger pitch angle, the angle of the chassis to the track surface.
Formula 1 chassis are mandated on the shape of the undertray. They are not allowed to use but a small section of the undertray for this purpose. The wings then become less of a tuning aid and more of a downforce creator. This makes the window of oppurtunity much smaller in locating the right setup. The F1 chassis is very pitch sensitive as well. When the chassis changes angle under braking or turn in the resulting loss of downforce is much more than in CART.
Hope this helps. Also, check out
www.howstuffworks.com for more in depth on CART cars.