Deploying AD 2008 on 2 sites

Hello, 

I want to install AD 2008 on 2 geographically disperse sites. These 2 sites will be replica of each other and users will either access Site A or Site B. Both sites must have same identical users i.e. If user created on Site A, then it must replicate it on Site B. Once this will be successful, i will setup Exchange 2010 sp2 hosting in this architecture. 

Please suggest any link/blog to deploy AD on both sites and enable replication. 

Hasan