Windows Server Versions (Requirements)
I often forget half of this stuff so I thought it might be cunning to put some of it into a post. Sorry if it goes on a bit, some of the numbers change depending on your choice of Service Pack and it was easier to split that sort of thing into separate tables.
Windows Server 2003 SP1 Requirements and Limits
Windows Server 2003 SP2 / R2 Requirements and Limits
Windows Server 2008 Requirements and Limits
More information about Windows 2008 can be found here.