Even the debate over whether Internet sales are incremental business or just business that has shifted from another media source has never been settled. About the only thing the industry as a whole can agree on is that Internet sales, regardless of your definition of an Internet sale, are here to stay, so you might as well figure out how to measure and monitor them.
What are valid measuring sticks? You need to have traffic in order to generate leads and leads come before sales, so start by tallying the unique visitors to your site. Then measure leads. Over time, you should see some trends in traffic and leads generated from your own site, which will help guide you in creating search engine marketing campaigns.