I have two high-end aero bikes, which have a lot of cool-factor, but the speed difference is very small. Obviously positioning your body for better aerodynamics and biomechanics will make the greatest difference. Narrowing your elbows a half inch will make a greater difference than aero tubing. If you have a good, stiff round-tubed bike with a geometry that allows you to get aero, you could gain speed with an aero helmet, reducing your Q-factor (how far from the midline the pedals are), wearing more aero clothing, getting water bottle in the right positions, etc. Make sure your knees stay narrow in your pedaling motion and that your low back is not rounded. If you haven't had a real, professional bike fit by a biomechanics expert (not at a bike shop), that will cut far more off your bike split than aero tubes.
http://www.fitness-concepts.com/bike_fitting.htm

If you're looking for an objective quantification, go to AnalyticCycling.com

Ken

I agree with Ken, an aero frame really doesn't mean much and will not have a noticeable impact on your riding times UNLESS having a "cool" bike will get you to ride more then you will notice improved bike times. Fit is the most important factor and then any decent bike and good wheel set will get you where you want to go. For 99% of athletes our engine needs the work, not our bikes but getting a cool bike part sure is fun!