Creating AD Forest on Campus

I manage a medium size corporate branch office with 250 to 500 users. We’re Still running NT 4.0 DOMain but the Corporate is running AD. We’re planning of buying a new 2003 server and configure ADS in next few weeks. I’d like to know how to setup a Forest and use the existing DNS server the corporate uses. I’m NOT 100% sure of this plan. Any ideas, comments and suggestions are welcome.
Thanks.