I assume a state-of-the-art installation therefore nothing weird like DC and SQL-Server on the same machine. Think about an installation following the commendations of microsft and all the recommended settings on both servers in a windows 2008R2 domain with sql2008R2.
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Ideally, SQL Server should run on a dedicated server - no IIS, no DC, no other apps! This is primarily for security and performance reasons - memory, and disc IO particularly - but it also makes troubleshooting easier. If this is not about SQL Server Express, the licencing costs of SQL Server surely justify a dedicated box.
In clustered SQL Server environments, the installer prohibits SQL Server being installed on a box that is a DC.