Active Directory Server

Server 2008 Enterprise Setting up a Active Directory to Use for my Business to control Users And machines on the network and DFS Shares For Different Users "Security" also setting up a Site VPN from other Sites, Now, i guess one of my Main Question is i already have a Hosted Domain on the Web xyzcorp.com do i need to use xyzcorp.com Hosting DNS Servers IP for my DNS server in the AD or Do i use my ISP DNS Settings,
i just have Mega Tons of Questions on this scenario , any help would be great, "technically" not a TOP level domain that already xyzcorp.com, so it would be buis.xyzcorp.com