Consumer-driven change?

In one of my gastronomy classes, we are currently studying classes and political influence. The current issue I am trying to wrap my head around is this: proponents of market economics (and western thought in general) suggest that consumer-driven choice can affect change in a society. For instance, if we want to change our food system to something healthier and more sustainable, "voting with our dollars" is the way to go. Basically, the theory is that by giving money to those producers that are aligning themselves with our ideals that those ideals will take over in a market setting. This is often done through shopping at very conscientious markets like Whole Foods or Trader Joe's. We assume that producers will eventually change their offerings to meet our monetary demands.

However, as we do so, a question arises. Are we really affecting change or merely setting ourselves and our class (well-off financially, educated, and frequently white) apart from general consumption? Will Whole Foods really change american buying patterns or just help us feel superior in our organic, sustainable, educated, white store? Will our consumer decisions do anything for the less well-heeled? Can systemic change be brought about effectively through individual actions or is a more holistic and politic approach called for?