how much of a difference does bike weight make when trying to maximize the bike's top speed? Today I was at a bike shop looking at single speed bikes. I rode one, but didn't buy it because I like to bike fast, and this bike must have weighed 2x more than my aluminum road bike (the bike was steel).

The store owner told me that the bike weight will not have as great of an effect on my top speed as many other things both on the bike and things I could change about how I ride.

Is he right or wrong? Does bike weight make a big difference or is it just very good marketing?
The weight of a bike has virtually no effect top speed. It takes very nearly the same amount of effort to maintain a speed of 30 mph on a 20 lb bike and a 30 lb bike on flat ground. The biggest factors that come into play at high speeds are aerodynamics and rolling resistance ( slick, efficient tires ).

http://velonews.competitor.com/2012/05/bikes-and-tech/technical-faq/tech-faq-does-bike-weight-matter_220429

That said, is top speed what you're really worried about? Probably not. Even if you like to bike fast, I truly, truly doubt you should be concerned with top speed. You'll probably enjoy a bike that accelerates quickly and feels nimble. Unless you live in a flat, remote area, in which case you probably wouldn't want a single speed, you're probably going to be accelerating and maneuvering more than you are achieving top speed. And again, these factors don't necessarily correlate with the weight of the bike.