Active directory

thats a link to the offical planing guide from MS http://www.microsoft.com/en-us/download/details.aspx?id=8133. I think you will find there some answeres to your questions.
Please refer all these links for your best answer

http://technet.microsoft.com/en-us/library/bb727085.aspx

Active directory best practices

http://technet.microsoft.com/en-us/library/cc778219(v=ws.10).aspx

Active directory best analyzer

http://technet.microsoft.com/en-us/library/dd378893(v=ws.10).aspx

Active directory virtualization best practices

http://www.techrepublic.com/blog/networking/active-directory-virtualization-best-practices/2433

And active directory tutorial

http://www.activedirectorytutorial.net/