Figuring out Microsoft licensing is easy. Call CDW and expect a wrong answer in about a week.

Seriously though, Microsoft licensing has gotten more and more difficult. You may purchase Microsoft products from a generalist like CDW or from a specific hardware vendor like HP or directly from Microsoft under a developer agreement. You may need to purchase Software Assurance to get better pricing, and although sometimes there's a discount for buying large numbers of licenses at once, that may not be readily apparent by looking at CDW's site.

Microsoft has kindly developed a video series that explains the complexities of licensing their products for virtual use, but has kept it vague enough that you'll still need to speak with a Microsoft licensing expert.

http://www.microsoft.com/licensing/about-licensing/virtualization.aspx