This TechNet site on Exchange versions and compatibility is really good.
Check out some of these grids.
Supported Operating System Platforms
Beneath the above grid are Active Directory version compatibility grids and more.
And there are a lot more.
The grids have not been updated with Exchange 2010 information yet, but that should be coming down the pipe pretty soon.
This is one site that we should have bookmarked for sure!
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book