Random thoughts and helpful IT tidbits
I have been asked this many times, what roles do we put where in AD? Most of the time, the environments are a standard 2 domain controller environment with a single site. Well, you can find a good technet article but need to dissect it. http://support.microsoft.com/kb/223346
For arguments sake, we will select DC01 and DC02 as the two domain controllers. DC01 will be the default main domain controller
So, all but one role is reccomended to go to one domain controller, while the others go to the second. Remember, if the primary fails, you can always seize the roles and remove the old primary, and rebuild the backup.