MS Exchange Server 2000

I am looking to implement an Exchange Server 2000 server into a Windows 2000 environment with three Domain Controller at different locations and different subnets, and one ISA Server. How would you recommend that I setup the ISA Server and Exchange Servers to achieve the best possible results? I would prefer to have everything external to my intranet forwarded through the ISA Server and to have external emails forwarded to clients mailboxes. Also, what sre your recommendations as far as literatures go?