ISA Installation and placing (what is the best practice)?

Hi there

I’m using ISA 2006 on my network.ISA machine is a stand alone machine right now with win2k3 as OS and configured as a workgroup.

there is another machine serving as a domain controller running win2k3 providing database connectivity,DNS,DHCP, and AD.

I want to know that what should be my strategy to configure ISA Server 2006, by that I mean I want this ISA server to be a part of the domain that I already have, so What would be the best practice to make this happen:

1. Should I make this ISA server child domain?
2. just change ISA server Primary DNS suffix?
3. or anyother best practice?

any help would be appreciated.
TIA
Cheers