Knowing what Microsoft you need to purchase can be very confusing. Here is a short summary of the version of Microsoft 2008 server and what the differences between the versions are:
2008 R2 Standard Edition: 1 physical server, up to 4 processors, 32 GB memory,1 virtual server
http://www.microsoft.com/windowsserver2008/en/us/2008-std.aspx
2008 R2 Enterprise Edition: 1 physical server, up to 8 processors, 2 TB memory, 4 virtual servers
http://www.microsoft.com/windowsserver2008/en/us/2008-ent.aspx
2008 R2 Data Center Edition: up to 64 processors, 256 cores, 2 TB memory, unlimited virtual servers
http://www.microsoft.com/windowsserver2008/en/us/2008-dc.aspx