Active Directory

I plan to implement Windows 2000 Active Directory for my company. Basically it will be a single forest and domain. I just got the information from my IT manager that we will merge with a new company ZTech, and we need to design and configure ZTech LAN to become a child domain (or a site) of our root AD. I found out that ZTech company already have Windows 2003 Active Directory in place since last year. My concern is what problems and impacts to ZTech’s Active Directory (domain) if I make it as a child domain of our new Windows 2003 Active Directory? What is the best solutin for this mess!! Thanks-