Windows 2008 R2 standard or enterprise

I want to know which version of Windows 2008 R2 for the domain controller that I have, that might be around 1000 users. What all kindof options will I need and then I will have to select Windows 2008 R2 Enterprise, apart from clustering doesnt counting? Is there any RADIUS client limitation or certificate server? Thanks.

I dont think that it matters if you use standard or enterprise edition for a DC itself, but you have some roles/features not available or complete, check here about the details, especially for CA:

http://www.microsoft.com/windowsserv...are-roles.aspx
http://www.microsoft.com/windowsserv...-features.aspx

I think for clustering you have atleast to use the enterprise or data center edition and for Radius I dont know about changes.