Some Important points about DAG to get you started:
DAG can have an incremental deployment, which means, you can have a highly available deployment even if initially you have installed the standalone mailbox servers. However, if you plan to do so, make sure that you are using Enterprise edition of Windows Server 2008 because DAG uses Windows Failover clustering in the backend.
DAG can comprise of maximum 16 mailbox servers hence there can be a maximum of 16 database copies across DAG.
DAG can span across Active Directory sites (physical locations).
DAG uses TCP socket (not SMB) to replicate logs hence it is possible to customize the port for DAG replication and compress the log shipping.