As said above. Tires and Wheels make a huge difference. Consensus among those who actually look at data seems to run along the lines of wheel aerodynamics making the biggest difference. Even at the expense of slightly higher overall weight. That said...a lightweight aero wheel with the appropriate tire inflated to the correct PSI would be ideal.
I've heard that the biggest performance advantage for the least money comes in tires/tire pressure.
Remember that anything you can do to clean up aerodynamics will allow you to be faster with the same output. Clothing, helmets, ride position (to a point), bottle cages, cable housing